Cyanophages (cyanobacterial viruses) are important agents of horizontal gene transfer among marine cyanobacteria, the numerically dominant photosynthetic organisms in the oceans. Some cyanophage genomes carry and express host-like photosynthesis genes, presumably to augment the host photosynthetic machinery during infection. To study the prevalence and evolutionary dynamics of this phenomenon, 33 cultured cyanophages of known family and host range and viral DNA from field samples were screened for the presence of two core photosystem reaction center genes,
psbD. Combining this expanded dataset with published data for nine other cyanophages, we found that 88% of the phage genomes contain
psbA, and 50% contain both
psbA gene was found in all myoviruses and
Prochlorococcus podoviruses, but could not be amplified from
Prochlorococcus siphoviruses or
Synechococcus podoviruses. Nearly all of the phages that encoded both
psbD had broad host ranges. We speculate that the presence or absence of
psbA in a phage genome may be determined by the length of the latent period of infection. Whether it also carries
psbD may reflect constraints on coupling of viral- and host-encoded PsbA–PsbD in the photosynthetic reaction center across divergent hosts. Phylogenetic clustering patterns of these genes from cultured phages suggest that whole genes have been transferred from host to phage in a discrete number of events over the course of evolution (four for
psbA, and two for
psbD), followed by horizontal and vertical transfer between cyanophages. Clustering patterns of
Synechococcus cells were inconsistent with other molecular phylogenetic markers, suggesting genetic exchanges involving
Synechococcus lineages. Signatures of intragenic recombination, detected within the cyanophage gene pool as well as between hosts and phages in both directions, support this hypothesis. The analysis of cyanophage
psbD genes from field populations revealed significant sequence diversity, much of which is represented in our cultured isolates. Collectively, these findings show that photosynthesis genes are common in cyanophages and that significant genetic exchanges occur from host to phage, phage to host, and within the phage gene pool. This generates genetic diversity among the phage, which serves as a reservoir for their hosts, and in turn influences photosystem evolution.
Analysis of 33 cultured cyanophages of known family and host range, as well as viral DNA from field samples, reveals the prevalence of photosynthesis genes in cyanophages and demonstrates significant genetic exchanges between host and phage.
T4-like myoviruses are ubiquitous, and their genes are among the most abundant documented in ocean systems. Here we compare 26 T4-like genomes, including 10 from non-cyanobacterial myoviruses, and 16 from marine cyanobacterial myoviruses (cyanophages) isolated on diverse Prochlorococcus or Synechococcus hosts. A core genome of 38 virion construction and DNA replication genes was observed in all 26 genomes, with 32 and 25 additional genes shared among the non-cyanophage and cyanophage subsets, respectively. These hierarchical cores are highly syntenic across the genomes, and sampled to saturation. The 25 cyanophage core genes include six previously described genes with putative functions (psbA, mazG, phoH, hsp20, hli03, cobS), a hypothetical protein with a potential phytanoyl-CoA dioxygenase domain, two virion structural genes, and 16 hypothetical genes. Beyond previously described cyanophage-encoded photosynthesis and phosphate stress genes, we observed core genes that may play a role in nitrogen metabolism during infection through modulation of 2-oxoglutarate. Patterns among non-core genes that may drive niche diversification revealed that phosphorus-related gene content reflects source waters rather than host strain used for isolation, and that carbon metabolism genes appear associated with putative mobile elements. As well, phages isolated on Synechococcus had higher genome-wide %G+C and often contained different gene subsets (e.g. petE, zwf, gnd, prnA, cpeT) than those isolated on Prochlorococcus. However, no clear diagnostic genes emerged to distinguish these phage groups, suggesting blurred boundaries possibly due to cross-infection. Finally, genome-wide comparisons of both diverse and closely related, co-isolated genomes provide a locus-to-locus variability metric that will prove valuable for interpreting metagenomic data sets.
Prochlorococcus contributes significantly to ocean primary productivity. The link between primary productivity and iron in specific ocean regions is well established and iron limitation of Prochlorococcus cell division rates in these regions has been shown. However, the extent of ecotypic variation in iron metabolism among Prochlorococcus and the molecular basis for differences is not understood. Here, we examine the growth and transcriptional response of Prochlorococcus strains, MED4 and MIT9313, to changing iron concentrations. During steady state, MIT9313 sustains growth at an order-of-magnitude lower iron concentration than MED4. To explore this difference, we measured the whole-genome transcriptional response of each strain to abrupt iron starvation and rescue. Only four of the 1159 orthologs of MED4 and MIT9313 were differentially expressed in response to iron in both strains. However, in each strain, the expression of over a hundred additional genes changed, many of which are in labile genomic regions, suggesting a role for lateral gene transfer in establishing diversity of iron metabolism among Prochlorococcus. Furthermore, we found that MED4 lacks three genes near the iron-deficiency-induced gene (idiA) that are present and induced by iron stress in MIT9313. These genes are interesting targets for studying the adaptation of natural Prochlorococcus assemblages to local iron conditions as they show more diversity than other genomic regions in environmental metagenomic databases.
cyanobacteria; iron; transcriptome
Prochlorococcus, an abundant phototroph in the oceans, are infected by members of three families of viruses: myo-, podo- and siphoviruses. Genomes of myo- and podoviruses isolated on Prochlorococcus contain DNA replication machinery and virion structural genes homologous to those from coliphages T4 and T7 respectively. They also contain a suite of genes of cyanobacterial origin, most notably photosynthesis genes, which are expressed during infection and appear integral to the evolutionary trajectory of both host and phage. Here we present the first genome of a cyanobacterial siphovirus, P-SS2, which was isolated from Atlantic slope waters using a Prochlorococcus host (MIT9313). The P-SS2 genome is larger than, and considerably divergent from, previously sequenced siphoviruses. It appears most closely related to lambdoid siphoviruses, with which it shares 13 functional homologues. The ∼108 kb P-SS2 genome encodes 131 predicted proteins and notably lacks photosynthesis genes which have consistently been found in other marine cyanophage, but does contain 14 other cyanobacterial homologues. While only six structural proteins were identified from the genome sequence, 35 proteins were detected experimentally; these mapped onto capsid and tail structural modules in the genome. P-SS2 is potentially capable of integration into its host as inferred from bioinformatically identified genetic machinery int, bet, exo and a 53 bp attachment site. The host attachment site appears to be a genomic island that is tied to insertion sequence (IS) activity that could facilitate mobility of a gene involved in the nitrogen-stress response. The homologous region and a secondary IS-element hot-spot in Synechococcus RS9917 are further evidence of IS-mediated genome evolution coincident with a probable relic prophage integration event. This siphovirus genome provides a glimpse into the biology of a deep-photic zone phage as well as the ocean cyanobacterial prophage and IS element ‘mobilome’.
Marine photosynthesis is largely driven by cyanobacteria, namely Synechococcus and Prochlorococcus. Genes encoding for photosystem (PS) I and II reaction centre proteins are found in cyanophages and are believed to increase their fitness. Two viral PSI gene arrangements are known, psaJF→C→A→B→K→E→D and psaD→C→A→B. The shared genes between these gene cassettes and their encoded proteins are distinguished by %G + C and protein sequence respectively. The data on the psaD→C→A→B gene organization were reported from only two partial gene cassettes coming from Global Ocean Sampling stations in the Pacific and Indian oceans. Now we have extended our search to 370 marine stations from six metagenomic projects. Genes corresponding to both PSI gene arrangements were detected in the Pacific, Indian and Atlantic oceans, confined to a strip along the equator (30°N and 30°S). In addition, we found that the predicted structure of the viral PsaA protein from the psaD→C→A→B organization contains a lumenal loop conserved in PsaA proteins from Synechococcus, but is completely absent in viral PsaA proteins from the psaJF→C→A→B→K→E→D gene organization and most Prochlorococcus strains. This may indicate a co‐evolutionary scenario where cyanophages containing either of these gene organizations infect cyanobacterial ecotypes biogeographically restricted to the 30°N and 30°S equatorial strip.
Large swaths of the nutrient-poor surface ocean are dominated numerically by cyanobacteria (Prochlorococcus), cyanobacterial viruses (cyanophage), and alphaproteobacteria (SAR11). How these groups thrive in the diverse physicochemical environments of different oceanic regions remains poorly understood. Comparative metagenomics can reveal adaptive responses linked to ecosystem-specific selective pressures. The Red Sea is well-suited for studying adaptation of pelagic-microbes, with salinities, temperatures, and light levels at the extreme end for the surface ocean, and low nutrient concentrations, yet no metagenomic studies have been done there. The Red Sea (high salinity, high light, low N and P) compares favorably with the Mediterranean Sea (high salinity, low P), Sargasso Sea (low P), and North Pacific Subtropical Gyre (high light, low N). We quantified the relative abundance of genetic functions among Prochlorococcus, cyanophage, and SAR11 from these four regions. Gene frequencies indicate selection for phosphorus acquisition (Mediterranean/Sargasso), DNA repair and high-light responses (Red Sea/Pacific Prochlorococcus), and osmolyte C1 oxidation (Red Sea/Mediterranean SAR11). The unexpected connection between salinity-dependent osmolyte production and SAR11 C1 metabolism represents a potentially major coevolutionary adaptation and biogeochemical flux. Among Prochlorococcus and cyanophage, genes enriched in specific environments had ecotype distributions similar to nonenriched genes, suggesting that inter-ecotype gene transfer is not a major source of environment-specific adaptation. Clustering of metagenomes using gene frequencies shows similarities in populations (Red Sea with Pacific, Mediterranean with Sargasso) that belie their geographic distances. Taken together, the genetic functions enriched in specific environments indicate competitive strategies for maintaining carrying capacity in the face of physical stressors and low nutrient availability.
Cyanophage; metagenomics; osmolyte; Pelagibacter; population genomics; Prochlorococcus; SAR11
Podoviruses are among the major viral groups that infect marine picocyanobacteria Prochlorococcus and Synechococcus. Here, we reported the genome sequences of five Synechococcus podoviruses isolated from the estuarine environment, and performed comparative genomic and phylogenomic analyses based on a total of 20 cyanopodovirus genomes. The genomes of all the known marine cyanopodoviruses are highly syntenic. A pan-genome of 349 clustered orthologous groups was determined, among which 15 were core genes. These core genes make up nearly half of each genome in length, reflecting the high level of genome conservation among this cyanophage type. The whole genome phylogenies based on concatenated core genes and gene content were highly consistent and confirmed the separation of two discrete marine cyanopodovirus clusters MPP-A and MPP-B. The genomes within cluster MPP-B grouped into subclusters mainly corresponding to Prochlorococcus or Synechococcus host types. Auxiliary metabolic genes tend to occur in a specific phylogenetic group of these cyanopodoviruses. All the MPP-B phages analyzed here encode the photosynthesis gene psbA, which are absent in all the MPP-A genomes thus far. Interestingly, all the MPP-B and two MPP-A Synechococcus podoviruses encode the thymidylate synthase gene thyX, while at the same genome locus all the MPP-B Prochlorococcus podoviruses encode the transaldolase gene talC. Both genes are hypothesized to have the potential to facilitate the biosynthesis of deoxynucleotide for phage replication. Inheritance of specific functional genes could be important to the evolution and ecological fitness of certain cyanophage genotypes. Our analyses demonstrate that cyanopodoviruses of estuarine and oceanic origins share a conserved core genome and suggest that accessory genes may be related to environmental adaptation.
Many of the cyanobacterial species found in marine and saline environments have a gene encoding a putative nitrite transporter of the formate/nitrite transporter (FNT) family. The presumed function of the gene (designated nitM) was confirmed by functional expression of the gene from the coastal marine species Synechococcus sp. strain PCC7002 in the nitrite-transport-less mutant (NA4) of the freshwater cyanobacterium Synechococcus elongatus strain PCC7942. The NitM-mediated nitrite uptake showed an apparent Km (NO2−) of about 8 μM and was not inhibited by nitrate, cyanate or formate. Of the nitM orthologs from the three oceanic cyanobacterial species, which are classified as α-cyanobacteria on the basis of the occurrence of Type 1a RuBisCO, the one from Synechococcus sp. strain CC9605 conferred nitrite uptake activity on NA4, but those from Synechococcus sp. strain CC9311 and Prochlorococcus
marinus strain MIT9313 did not. A strongly conserved hydrophilic amino acid sequence was found at the C-termini of the deduced NitM sequences from α-cyanobacteria, with a notable exception of the Synechococcus sp. strain CC9605 NitM protein, which entirely lacked the C-terminal amino acids. The C-terminal sequence was not conserved in the NitM proteins from β-cyanobacteria carrying the Type 1b RuBisCO, including the one from Synechococcus sp. strain PCC7002. Expression of the truncated nitM genes from Synechococcus sp. strain CC9311 and Prochlorococcus
marinus strain MIT9313, encoding the proteins lacking the conserved C-terminal region, conferred nitrite uptake activity on the NA4 mutant, indicating that the C-terminal region of α-cyanobacterial NitM proteins inhibits the activity of the transporter.
marine cyanobacteria; nitrite; transporter
Marine cyanobacteria of the genera Prochlorococcus and Synechococcus are the most abundant photosynthetic prokaryotes in oceanic environments, and are key contributors to global CO2 fixation, chlorophyll biomass and primary production. Cyanophages, viruses infecting cyanobacteria, are a major force in the ecology of their hosts. These phages contribute greatly to cyanobacterial mortality, therefore acting as a powerful selective force upon their hosts. Phage reproduction is based on utilization of the host transcription and translation mechanisms; therefore, differences in the G+C genomic content between cyanophages and their hosts could be a limiting factor for the translation of cyanophage genes. On the basis of comprehensive genomic analyses conducted in this study, we suggest that cyanophages of the Myoviridae family, which can infect both Prochlorococcus and Synechococcus, overcome this limitation by carrying additional sets of tRNAs in their genomes accommodating AT-rich codons. Whereas the tRNA genes are less needed when infecting their Prochlorococcus hosts, which possess a similar G+C content to the cyanophage, the additional tRNAs may increase the overall translational efficiency of their genes when infecting a Synechococcus host (with high G+C content), therefore potentially enabling the infection of multiple hosts.
codon usage; cross-infectivity; marine cyanophages; Prochlorococcus; Synechococcus; tRNA
Prochlorococcus is a marine cyanobacterium that numerically dominates the mid-latitude oceans and is the smallest known oxygenic phototroph. Numerous isolates from diverse areas of the world's oceans have been studied and shown to be physiologically and genetically distinct. All isolates described thus far can be assigned to either a tightly clustered high-light (HL)-adapted clade, or a more divergent low-light (LL)-adapted group. The 16S rRNA sequences of the entire Prochlorococcus group differ by at most 3%, and the four initially published genomes revealed patterns of genetic differentiation that help explain physiological differences among the isolates. Here we describe the genomes of eight newly sequenced isolates and combine them with the first four genomes for a comprehensive analysis of the core (shared by all isolates) and flexible genes of the Prochlorococcus group, and the patterns of loss and gain of the flexible genes over the course of evolution. There are 1,273 genes that represent the core shared by all 12 genomes. They are apparently sufficient, according to metabolic reconstruction, to encode a functional cell. We describe a phylogeny for all 12 isolates by subjecting their complete proteomes to three different phylogenetic analyses. For each non-core gene, we used a maximum parsimony method to estimate which ancestor likely first acquired or lost each gene. Many of the genetic differences among isolates, especially for genes involved in outer membrane synthesis and nutrient transport, are found within the same clade. Nevertheless, we identified some genes defining HL and LL ecotypes, and clades within these broad ecotypes, helping to demonstrate the basis of HL and LL adaptations in Prochlorococcus. Furthermore, our estimates of gene gain events allow us to identify highly variable genomic islands that are not apparent through simple pairwise comparisons. These results emphasize the functional roles, especially those connected to outer membrane synthesis and transport that dominate the flexible genome and set it apart from the core. Besides identifying islands and demonstrating their role throughout the history of Prochlorococcus, reconstruction of past gene gains and losses shows that much of the variability exists at the “leaves of the tree,” between the most closely related strains. Finally, the identification of core and flexible genes from this 12-genome comparison is largely consistent with the relative frequency of Prochlorococcus genes found in global ocean metagenomic databases, further closing the gap between our understanding of these organisms in the lab and the wild.
Prochlorococcus—the most abundant photosynthetic microbe living in the vast, nutrient-poor areas of the ocean—is a major contributor to the global carbon cycle. Prochlorococcus is composed of closely related, physiologically distinct lineages whose differences enable the group as a whole to proliferate over a broad range of environmental conditions. We compare the genomes of 12 strains of Prochlorococcus representing its major lineages in order to identify genetic differences affecting the ecology of different lineages and their evolutionary origin. First, we identify the core genome: the 1,273 genes shared among all strains. This core set of genes encodes the essentials of a functional cell, enabling it to make living matter out of sunlight and carbon dioxide. We then create a genomic tree that maps the gain and loss of non-core genes in individual strains, showing that a striking number of genes are gained or lost even among the most closely related strains. We find that lost and gained genes commonly cluster in highly variable regions called genomic islands. The level of diversity among the non-core genes, and the number of new genes added with each new genome sequenced, suggest far more diversity to be discovered.
Prochlorococcus is a genus of abundant and ecologically important marine cyanobacteria. Here, we present a comprehensive comparison of the structure and composition of the transcriptomes of two Prochlorococcus strains, which, despite their similarities, have adapted their gene pool to specific environmental constraints. We present genome-wide maps of transcriptional start sites (TSS) for both organisms, which are representatives of the two most diverse clades within the two major ecotypes adapted to high- and low-light conditions, respectively. Our data suggest antisense transcription for three-quarters of all genes, which is substantially more than that observed in other bacteria. We discovered hundreds of TSS within genes, most notably within 16 of the 29 prochlorosin genes, in strain MIT9313. A direct comparison revealed very little conservation in the location of TSS and the nature of non-coding transcripts between both strains. We detected extremely short 5′ untranslated regions with a median length of only 27 and 29 nt for MED4 and MIT9313, respectively, and for 8% of all protein-coding genes the median distance to the start codon is only 10 nt or even shorter. These findings and the absence of an obvious Shine–Dalgarno motif suggest that leaderless translation and ribosomal protein S1-dependent translation constitute alternative mechanisms for translation initiation in Prochlorococcus. We conclude that genome-wide antisense transcription is a major component of the transcriptional output from these relatively small genomes and that a hitherto unrecognized high degree of complexity and variability of gene expression exists in their transcriptional architecture.
antisense RNA; cyanobacteria; dRNA-seq; Prochlorococcus; transcriptomics; prochlorosin
Cyanophage infecting the marine cyanobacteria Prochlorococcus and Synechococcus require light and host photosystem activity for optimal reproduction. Many cyanophages encode multiple photosynthetic electron transport (PET) proteins, which are presumed to maintain electron flow and produce ATP and NADPH for nucleotide biosynthesis and phage genome replication. However, evidence suggests phage augment NADPH production via the pentose phosphate pathway (PPP), thus calling into question the need for NADPH production by PET. Genes implicated in cyclic PET have since been identified in cyanophage genomes. It remains an open question which mode of PET, cyclic or linear, predominates in infected cyanobacteria, and thus whether the balance is towards producing ATP or NADPH. We sequenced transcriptomes of a cyanophage (P-HM2) and its host (Prochlorococcus MED4) throughout infection in the light or in the dark, and analyzed these data in the context of phage replication and metabolite measurements. Infection was robust in the light, but phage were not produced in the dark. Host gene transcripts encoding high-light inducible proteins and two terminal oxidases (plastoquinol terminal oxidase and cytochrome c oxidase)—implicated in protecting the photosynthetic membrane from light stress—were the most enriched in light but not dark infection. Among the most diminished transcripts in both light and dark infection was ferredoxin–NADP+ reductase (FNR), which uses the electron acceptor NADP+ to generate NADPH in linear photosynthesis. The phage gene for CP12, which putatively inhibits the Calvin cycle enzyme that receives NADPH from FNR, was highly expressed in light infection. Therefore, both PET production of NADPH and its consumption by carbon fixation are putatively repressed during phage infection in light. Transcriptomic evidence is thus consistent with cyclic photophosphorylation using oxygen as the terminal electron acceptor as the dominant mode of PET under infection, with ATP from PET and NADPH from the PPP producing the energy and reducing equivalents for phage nucleotide biosynthesis and replication.
The phylogeny and taxonomy of cyanobacteria is currently poorly understood due to paucity of reliable markers for identification and circumscription of its major clades.
A combination of phylogenomic and protein signature based approaches was used to characterize the major clades of cyanobacteria. Phylogenetic trees were constructed for 44 cyanobacteria based on 44 conserved proteins. In parallel, Blastp searches were carried out on each ORF in the genomes of Synechococcus WH8102, Synechocystis PCC6803, Nostoc PCC7120, Synechococcus JA-3-3Ab, Prochlorococcus MIT9215 and Prochlor. marinus subsp. marinus CCMP1375 to identify proteins that are specific for various main clades of cyanobacteria. These studies have identified 39 proteins that are specific for all (or most) cyanobacteria and large numbers of proteins for other cyanobacterial clades. The identified signature proteins include: (i) 14 proteins for a deep branching clade (Clade A) of Gloebacter violaceus and two diazotrophic Synechococcus strains (JA-3-3Ab and JA2-3-B'a); (ii) 5 proteins that are present in all other cyanobacteria except those from Clade A; (iii) 60 proteins that are specific for a clade (Clade C) consisting of various marine unicellular cyanobacteria (viz. Synechococcus and Prochlorococcus); (iv) 14 and 19 signature proteins that are specific for the Clade C Synechococcus and Prochlorococcus strains, respectively; (v) 67 proteins that are specific for the Low B/A ecotype Prochlorococcus strains, containing lower ratio of chl b/a2 and adapted to growth at high light intensities; (vi) 65 and 8 proteins that are specific for the Nostocales and Chroococcales orders, respectively; and (vii) 22 and 9 proteins that are uniquely shared by various Nostocales and Oscillatoriales orders, or by these two orders and the Chroococcales, respectively. We also describe 3 conserved indels in flavoprotein, heme oxygenase and protochlorophyllide oxidoreductase proteins that are specific for either Clade C cyanobacteria or for various subclades of Prochlorococcus. Many other conserved indels for cyanobacterial clades have been described recently.
These signature proteins and indels provide novel means for circumscription of various cyanobacterial clades in clear molecular terms. Their functional studies should lead to discovery of novel properties that are unique to these groups of cyanobacteria.
Myoviruses and podoviruses that infect cyanobacteria are the two major groups of marine cyanophages, but little is known of how their phylogenetic lineages are distributed in different habitats. In this study, we analyzed the phylogenetic relationships of cyanopodoviruses and cyanomyoviruses based on the existing genomes. The 28 cyanomyoviruses were classified into four clusters (I to IV), and 19 of the 20 cyanopodoviruses were classified into two clusters, MPP-A and MPP-B, with four subclusters within cluster MPP-B. These genomes were used to recruit cyanophage-like fragments from microbial and viral metagenomes to estimate the relative abundances of these cyanophage lineages. Our results showed that cyanopodoviruses and cyanomyoviruses are both abundant in various marine environments and that clusters MPP-B, II and III appear to be the most dominant lineages. Cyanopodoviruses and cluster I and IV cyanomyoviruses exhibited habitat-related variability in their relative levels of abundance, while cluster II and III cyanomyoviruses appeared to be consistently dominant in various habitats. Multivariate analyses showed that reads that mapped to Synechococcus phages and Prochlorococcus phages had distinct distribution patterns that were significantly correlated to those of Synechococcus and Prochlorococcus, respectively. The Mantel test also revealed a strong correlation between the community compositions of cyanophages and picocyanobacteria. Given that cyanomyoviruses tend to have a broad host range and some can cross-infect Synechococcus and Prochlorococcus, while cyanopodoviruses are commonly host specific, the observation that their community compositions both correlated significantly with that of picocyanobacteria was unexpected. Although cyanomyoviruses and cyanopodoviruses differ in host specificity, their biogeographic distributions are likely both constrained by the picocyanobacterial community.
Many cyanophage isolates which infect the marine cyanobacteria Synechococcus spp. and Prochlorococcus spp. contain a gene homologous to psbA, which codes for the D1 protein involved in photosynthesis. In the present study, cyanophage psbA gene fragments were readily amplified from freshwater and marine samples, confirming their widespread occurrence in aquatic communities. Phylogenetic analyses demonstrated that sequences from freshwaters have an evolutionary history that is distinct from that of their marine counterparts. Similarly, sequences from cyanophages infecting Prochlorococcus and Synechococcus spp. were readily discriminated, as were sequences from podoviruses and myoviruses. Viral psbA sequences from the same geographic origins clustered within different clades. For example, cyanophage psbA sequences from the Arctic Ocean fell within the Synechococcus as well as Prochlorococcus phage groups. Moreover, as psbA sequences are not confined to a single family of phages, they provide an additional genetic marker that can be used to explore the diversity and evolutionary history of cyanophages in aquatic environments.
Prochlorococcus, an extremely small cyanobacterium that is very abundant in the world's oceans, has a very streamlined genome. On average, these cells have about 2,000 genes and very few regulatory proteins. The limited capability of regulation is thought to be a result of selection imposed by a relatively stable environment in combination with a very small genome. Furthermore, only ten non-coding RNAs (ncRNAs), which play crucial regulatory roles in all forms of life, have been described in Prochlorococcus. Most strains also lack the RNA chaperone Hfq, raising the question of how important this mode of regulation is for these cells. To explore this question, we examined the transcription of intergenic regions of Prochlorococcus MED4 cells subjected to a number of different stress conditions: changes in light qualities and quantities, phage infection, or phosphorus starvation. Analysis of Affymetrix microarray expression data from intergenic regions revealed 276 novel transcriptional units. Among these were 12 new ncRNAs, 24 antisense RNAs (asRNAs), as well as 113 short mRNAs. Two additional ncRNAs were identified by homology, and all 14 new ncRNAs were independently verified by Northern hybridization and 5′RACE. Unlike its reduced suite of regulatory proteins, the number of ncRNAs relative to genome size in Prochlorococcus is comparable to that found in other bacteria, suggesting that RNA regulators likely play a major role in regulation in this group. Moreover, the ncRNAs are concentrated in previously identified genomic islands, which carry genes of significance to the ecology of this organism, many of which are not of cyanobacterial origin. Expression profiles of some of these ncRNAs suggest involvement in light stress adaptation and/or the response to phage infection consistent with their location in the hypervariable genomic islands.
Prochlorococcus is the most abundant phototroph in the vast, nutrient-poor areas of the ocean. It plays an important role in the ocean carbon cycle, and is a key component of the base of the food web. All cells share a core set of about 1,200 genes, augmented with a variable number of “flexible” genes. Many of the latter are located in genomic islands—hypervariable regions of the genome that encode functions important in differentiating the niches of “ecotypes.” Of major interest is how cells with such a small genome regulate cellular processes, as they lack many of the regulatory proteins commonly found in bacteria. We show here that contrary to the regulatory proteins, ncRNAs are present at levels typical of bacteria, revealing that they might have a disproportional regulatory role in Prochlorococcus—likely an adaptation to the extremely low-nutrient conditions of the open oceans, combined with the constraints of a small genome. Some of the ncRNAs were differentially expressed under stress conditions, and a high number of them were found to be associated with genomic islands, suggesting functional links between these RNAs and the response of Prochlorococcus to particular environmental challenges.
Prochlorococcus is a genus of marine cyanobacteria characterized by small cell and genome size, an evolutionary trend toward low GC content, the possession of chlorophyll b, and the absence of phycobilisomes. Whereas many shared derived characters define Prochlorococcus as a clade, many genome-based analyses recover them as paraphyletic, with some low-light adapted Prochlorococcus spp. grouping with marine Synechococcus. Here, we use 18 Prochlorococcus and marine Synechococcus genomes to analyze gene flow within and between these taxa. We introduce embedded quartet scatter plots as a tool to screen for genes whose phylogeny agrees or conflicts with the plurality phylogenetic signal, with accepted taxonomy and naming, with GC content, and with the ecological adaptation to high and low light intensities. We find that most gene families support high-light adapted Prochlorococcus spp. as a monophyletic clade and low-light adapted Prochlorococcus sp. as a paraphyletic group. But we also detect 16 gene families that were transferred between high-light adapted and low-light adapted Prochlorococcus sp. and 495 gene families, including 19 ribosomal proteins, that do not cluster designated Prochlorococcus and Synechococcus strains in the expected manner. To explain the observed data, we propose that frequent gene transfer between marine Synechococcus spp. and low-light adapted Prochlorococcus spp. has created a “highway of gene sharing” (Beiko RG, Harlow TJ, Ragan MA. 2005. Highways of gene sharing in prokaryotes. Proc Natl Acad Sci USA. 102:14332–14337) that tends to erode genus boundaries without erasing the Prochlorococcus-specific ecological adaptations.
marine cyanobacteria; horizontal gene transfer; introgression; quartet decomposition; supertree; genome evolution
The oceanic cyanobacteria Prochlorococcus are globally important, ecologically diverse primary producers. It is thought that their viruses (phages) mediate population sizes and affect the evolutionary trajectories of their hosts. Here we present an analysis of genomes from three Prochlorococcus phages: a podovirus and two myoviruses. The morphology, overall genome features, and gene content of these phages suggest that they are quite similar to T7-like (P-SSP7) and T4-like (P-SSM2 and P-SSM4) phages. Using the existing phage taxonomic framework as a guideline, we examined genome sequences to establish “core” genes for each phage group. We found the podovirus contained 15 of 26 core T7-like genes and the two myoviruses contained 43 and 42 of 75 core T4-like genes. In addition to these core genes, each genome contains a significant number of “cyanobacterial” genes, i.e., genes with significant best BLAST hits to genes found in cyanobacteria. Some of these, we speculate, represent “signature” cyanophage genes. For example, all three phage genomes contain photosynthetic genes (psbA, hliP) that are thought to help maintain host photosynthetic activity during infection, as well as an aldolase family gene (talC) that could facilitate alternative routes of carbon metabolism during infection. The podovirus genome also contains an integrase gene (int) and other features that suggest it is capable of integrating into its host. If indeed it is, this would be unprecedented among cultured T7-like phages or marine cyanophages and would have significant evolutionary and ecological implications for phage and host. Further, both myoviruses contain phosphate-inducible genes (phoH and pstS) that are likely to be important for phage and host responses to phosphate stress, a commonly limiting nutrient in marine systems. Thus, these marine cyanophages appear to be variations of two well-known phages—T7 and T4—but contain genes that, if functional, reflect adaptations for infection of photosynthetic hosts in low-nutrient oceanic environments.
An analysis of the genome sequences of three phages capable of infecting marine unicellular cyanobacteria Prochlorococcus reveals they are genetically complex with intriguing adaptations related to their oceanic environment
S-PM2 is a phage capable of infecting strains of unicellular cyanobacteria belonging to the genus Synechococcus. S-PM2, like other myoviruses infecting marine cyanobacteria, encodes a number of bacterial-like genes. Amongst these genes is one encoding a MazG homologue that is hypothesized to be involved in the adaption of the infected host for production of progeny phage.
This study focuses on establishing the occurrence of mazG homologues in other cyanophages isolated from different oceanic locations. Degenerate PCR primers were designed using the mazG gene of S-PM2. The mazG gene was found to be widely distributed and highly conserved among Synechococcus myoviruses and podoviruses from diverse oceanic provinces.
This study provides evidence of a globally connected cyanophage gene pool, the cyanophage mazG gene having a small effective population size indicative of rapid lateral gene transfer despite being present in a substantial fraction of cyanophage. The Prochlorococcus and Synechococcus phage mazG genes do not cluster with the host mazG gene, suggesting that their primary hosts are not the source of the mazG gene.
Prochlorococcus and Synechococcus are the most abundant photosynthetic organisms in oligotrophic waters and responsible for a significant percentage of the earth's primary production. Here we developed a method for metagenomic sequencing of sorted Prochlorococcus and Synechococcus populations using a transposon-based library preparation technique. First, we observed that the cell lysis technique and associated amount of input DNA had an important role in determining the DNA library quality. Second, we found that our transposon-based method provided a more even coverage distribution and matched more sequences of a reference genome than multiple displacement amplification, a commonly used method for metagenomic sequencing. We then demonstrated the method on Prochlorococcus and Synechococcus field populations from the Sargasso Sea and California Current isolated by flow cytometric sorting and found clear environmentally related differences in ecotype distributions and gene abundances. In addition, we saw a significant correspondence between metagenomic libraries sequenced with our technique and regular sequencing of bulk DNA. Our results show that this targeted method is a viable replacement for regular metagenomic approaches and will be useful for identifying the biogeography and genome content of specific marine cyanobacterial populations.
Prochlorococcus is the smallest oxygenic phototroph yet described. It numerically dominates the phytoplankton community in the mid-latitude oceanic gyres, where it has an important role in the global carbon cycle. The complete genomes of several Prochlorococcus strains have been sequenced, revealing that nearly half of the genes in each genome are of unknown function. Genetic methods, such as reporter gene assays and tagged mutagenesis, are critical to unveiling the functions of these genes. Here, we describe conditions for the transfer of plasmid DNA into Prochlorococcus strain MIT9313 by interspecific conjugation with Escherichia coli. Following conjugation, E. coli bacteria were removed from the Prochlorococcus cultures by infection with E. coli phage T7. We applied these methods to show that an RSF1010-derived plasmid will replicate in Prochlorococcus strain MIT9313. When this plasmid was modified to contain green fluorescent protein, we detected its expression in Prochlorococcus by Western blotting and cellular fluorescence. Further, we applied these conjugation methods to show that a mini-Tn5 transposon will transpose in vivo in Prochlorococcus. These genetic advances provide a basis for future genetic studies with Prochlorococcus, a microbe of ecological importance in the world's oceans.
Prochlorococcus is the numerically dominant phototroph in the oligotrophic subtropical ocean and carries out a significant fraction of marine primary productivity. Although field studies have provided evidence for nitrate uptake by Prochlorococcus, little is known about this trait because axenic cultures capable of growth on nitrate have not been available. Additionally, all previously sequenced genomes lacked the genes necessary for nitrate assimilation. Here we introduce three Prochlorococcus strains capable of growth on nitrate and analyze their physiology and genome architecture. We show that the growth of high-light (HL) adapted strains on nitrate is ∼17% slower than their growth on ammonium. By analyzing 41 Prochlorococcus genomes, we find that genes for nitrate assimilation have been gained multiple times during the evolution of this group, and can be found in at least three lineages. In low-light adapted strains, nitrate assimilation genes are located in the same genomic context as in marine Synechococcus. These genes are located elsewhere in HL adapted strains and may often exist as a stable genetic acquisition as suggested by the striking degree of similarity in the order, phylogeny and location of these genes in one HL adapted strain and a consensus assembly of environmental Prochlorococcus metagenome sequences. In another HL adapted strain, nitrate utilization genes may have been independently acquired as indicated by adjacent phage mobility elements; these genes are also duplicated with each copy detected in separate genomic islands. These results provide direct evidence for nitrate utilization by Prochlorococcus and illuminate the complex evolutionary history of this trait.
Marine photosynthesis is one of the major contributors to the global carbon cycle and the world's oxygen supply. This process is largely driven by cyanobacteria, namely Synechococcus and Prochlorococcus. Genes encoding photosystem-II (PSII) reaction center proteins are found in many cyanophage genomes, and are expressed during the infection of their hosts. On the basis of metagenomics, cyanophage photosystem-I (PSI) gene cassettes were recently discovered with two gene arrangements psaJF→C→A→B→K→E→D and psaD→C→A→B. It was suggested that the horizontal transfer of PSII and PSI genes is increasing phage fitness. To better understand their diversity, we designed degenerate primers to cover a wide diversity of organisms, and using PCR we targeted the psaC→A arrangement, which is unique to cyanophages cassettes. We examined viral concentrates from four islands in the Pacific Ocean and found samples containing the psaC→A arrangement. Analyses of the amplified viral psaA gene revealed six subgroups varying in their level of similarity and %G+C content, suggesting that the diversity of cyanophage PSI genes is greater than originally thought.
Cultured isolates of the marine cyanobacteria Prochlorococcus and Synechococcus vary widely in their pigment compositions and growth responses to light and nutrients, yet show greater than 96% identity in their 16S ribosomal DNA (rDNA) sequences. In order to better define the genetic variation that accompanies their physiological diversity, sequences for the 16S-23S rDNA internal transcribed spacer (ITS) region were determined in 32 Prochlorococcus isolates and 25 Synechococcus isolates from around the globe. Each strain examined yielded one ITS sequence that contained two tRNA genes. Dramatic variations in the length and G+C content of the spacer were observed among the strains, particularly among Prochlorococcus strains. Secondary-structure models of the ITS were predicted in order to facilitate alignment of the sequences for phylogenetic analyses. The previously observed division of Prochlorococcus into two ecotypes (called high and low-B/A after their differences in chlorophyll content) were supported, as was the subdivision of the high-B/A ecotype into four genetically distinct clades. ITS-based phylogenies partitioned marine cluster A Synechococcus into six clades, three of which can be associated with a particular phenotype (motility, chromatic adaptation, and lack of phycourobilin). The pattern of sequence divergence within and between clades is suggestive of a mode of evolution driven by adaptive sweeps and implies that each clade represents an ecologically distinct population. Furthermore, many of the clades consist of strains isolated from disparate regions of the world's oceans, implying that they are geographically widely distributed. These results provide further evidence that natural populations of Prochlorococcus and Synechococcus consist of multiple coexisting ecotypes, genetically closely related but physiologically distinct, which may vary in relative abundance with changing environmental conditions.
Cyanobacteria of the Synechococcus and Prochlorococcus genera are important contributors to photosynthetic productivity in the open oceans1-3. Here, using pre-existing metagenomic datasets from the global ocean sampling (GOS) expedition4 as well as from viral biomes5, we show the first evidence for the presence of photosystem I (PSI) genes in genomes of marine viruses that infect these marine cyanobacteria. Recently, core photosystem II (PSII) genes were identified in cyanophages; they were proposed to be functional in photosynthesis and in increasing viral fitness by supplementing the host production of these proteins6-9. The 7 cyanobacterial core PSI genes identified in this study, psaA, B, C, D, E, K and a unique J and F fusion, form a distinctive cluster in cyanophage genomes, suggestive of selection for a distinct function in virus life cycle. The existence of this PSI cluster was confirmed with overlapping and long PCR performed on environmental DNA from the Northern Line Islands. Potentially, the 7 proteins encoded by the viral genes are sufficient to form an intact monomeric PSI complex. Projection of viral predicted peptides on the cyanobacterial PSI crystal structure10 suggested that the viral-PSI components may provide a unique way of funneling reducing power from respiratory and other electron transfer chains to PSI.