Summary: Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus numerically dominate the picophytoplankton of the world ocean, making a key contribution to global primary production. Prochlorococcus was isolated around 20 years ago and is probably the most abundant photosynthetic organism on Earth. The genus comprises specific ecotypes which are phylogenetically distinct and differ markedly in their photophysiology, allowing growth over a broad range of light and nutrient conditions within the 45°N to 40°S latitudinal belt that they occupy. Synechococcus and Prochlorococcus are closely related, together forming a discrete picophytoplankton clade, but are distinguishable by their possession of dissimilar light-harvesting apparatuses and differences in cell size and elemental composition. Synechococcus strains have a ubiquitous oceanic distribution compared to that of Prochlorococcus strains and are characterized by phylogenetically discrete lineages with a wide range of pigmentation. In this review, we put our current knowledge of marine picocyanobacterial genomics into an environmental context and present previously unpublished genomic information arising from extensive genomic comparisons in order to provide insights into the adaptations of these marine microbes to their environment and how they are reflected at the genomic level.
Picocyanobacteria of the genus Synechococcus are important contributors to marine primary production and are ubiquitous in the world's oceans. This genus is genetically diverse, and at least 10 discrete lineages or clades have been identified phylogenetically. However, little if anything is known about the genetic attributes which characterize particular lineages or are unique to specific strains. Here, we used a suppression subtractive hybridization (SSH) approach to identify strain- and clade-specific genes in two well-characterized laboratory strains, Synechococcus sp. strain WH8103 (clade III) and Synechococcus sp. strain WH7803 (clade V). Among the genes that were identified as potentially unique to each strain were genes encoding proteins that may be involved in specific predator avoidance, including a glycosyltransferase in strain WH8103 and a permease component of an ABC-type polysaccharide/polyol phosphate export system in WH7803. During this work the genome of one of these strains, WH7803, became available. This allowed assessment of the number of false-positive sequences (i.e., sequences present in the tester genome) present among the SSH-enriched sequences. We found that approximately 9% of the WH8103 sequences were potential false-positive sequences, which demonstrated that caution should be used when this technology is used to assess genomic differences in genetically similar bacterial strains.
After infection of swine with porcine reproductive and respiratory syndrome virus (PRRSV), there is a rapid rise of PRRSV-specific nonneutralizing antibodies (NNA), while neutralizing antibodies (NA) are detectable not sooner than 3 weeks later. To characterize neutralizing epitopes, we selected phages from a 12-mer phage display library using anti-PRRSV neutralizing monoclonal antibody (MAb) ISU25-C1. In addition, phages carrying peptides recognized by swine antibodies with high seroneutralizing titer were isolated after subtracting from the library those clones binding to swine anti-PRRSV serum with no neutralizing activity. Two epitopes located in the ectodomain of PRRSV GP5 were identified. One of these epitopes, which we named epitope B, was recognized both by neutralizing MAb ISU25-C1 and swine neutralizing serum (NS) but not by swine nonneutralizing serum (NNS), indicating that it is a neutralizing epitope. Epitope B is sequential, conserved among isolates, and not immunodominant. Antibodies directed against it are detected in serum late after infection. In contrast, the other epitope, which we named epitope A, is hypervariable and immunodominant. Antibodies against it appear early after infection with PRRSV. This epitope is recognized by swine NNA but is not recognized by either neutralizing MAb ISU25-C1 or swine NA, indicating that it is not involved in PRRSV neutralization. During infection with PRRSV, epitope A may act as a decoy, eliciting most of the antibodies directed to GP5 and delaying the induction of NA against epitope B for at least 3 weeks. These results are relevant to the design of vaccines against PRRSV.
Heparin-binding epidermal growth factor (HB-EGF) gene transcription is rapidly activated in NIH 3T3 cells transformed by oncogenic Ras and Raf and mediates the autocrine activation of the c-Jun N-terminal kinases (JNKs) observed in these cells. A 1.7-kb fragment of the promoter of the murine HB-EGF gene linked to a luciferase reporter was strongly induced following activation of deltaRaf-1:ER, a conditionally active form of oncogenic human Raf-1. Promoter activation by deltaRaf-1:ER required a composite AP-1/Ets transcription factor binding site located between bp -974 and -988 upstream of the translation initiation site. In vivo genomic footprinting indicated that the basal level of occupancy of this composite AP-1/Ets element increased following deltaRaf-1:ER activation. Cotransfection of Ets-2 and p44 mitogen-activated protein (MAP) kinase expression vectors strongly potentiated HB-EGF promoter activation in response to deltaRaf-1:ER. Potentiated activation required both p44 MAP kinase catalytic activity and threonine 72 in the Pointed domain of Ets-2. Biochemical assays demonstrated the ability of the p42 and p44 MAP kinases to phosphorylate Ets-2 on threonine 72. Importantly, in intact cells, the kinetics of phosphorylation of Ets-2 on this residue closely mirror the activation of the p42 and p44 MAP kinases and the observed onset of HB-EGF gene transcription following deltaRaf-1:ER activation. These data firmly establish Ets-2 as a direct target of the Raf-MEK-MAP kinase signaling pathway and strongly implicate Ets-2 in the regulation of HB-EGF gene expression.
The colony stimulating factor-1 receptor (CSF-1R) affects mitogenic growth and gene expression in NIH 3T3 cells through signaling pathways that require the products of the c-ras and c-myc proto-oncogenes. In this work we tested the hypothesis that there is direct communication between the Ras and Myc pathways. In transient transfection assays Ras increased by 5-fold transcriptional transactivation by chimeric c-Myc-Gal4 proteins. A constitutive active form of the CSF-1R also stimulated this activity and co-expression of a dominant negative ras gene ablated receptor stimulation. Deletion analysis of the c-Myc N-terminal region demonstrated that amino acid residues between positions 92 and 143 are the targets for Ras action. Transactivation by chimeric Myc proteins that were stably expressed could be transiently enhanced by either CSF-1 or serum, with peak activity occurring 2 h after mitogen stimulation. The steady-state levels of the chimeric c-Myc transactivators were increased following stimulation with CSF-1 or serum, but this increase in steady-state protein level did not strictly correlate with the increase in transactivation activity. Thus, Ras signaling may directly affect the activity of the c-Myc N-terminal region.
The Ras oncogene products regulate the expression of genes in transformed cells, and members of the Ets family of transcription factors have been implicated in this process. To determine which Ets factors are the targets of Ras signaling pathways, the abilities of several Ets factors to activate Ras-responsive enhancer (RRE) reporters in the presence of oncogenic Ras were examined. In transient transfection assay, reporters containing RREs composed of Ets-AP-1 binding sites could be activated 30-fold in NIH 3T3 fibroblasts and 80-fold in the macrophage-like line RAW264 by the combination of Ets1 or Ets2 and Ras but not by several other Ets factors that were tested in the assay. Ets2 and Ras also superactivated an RRE composed of Ets-Ets binding sites, but the Ets-responsive promoter of the c-fms gene was not superactivated. Mutation of a threonine residue to alanine in the conserved amino-terminal regions of Ets1 and Ets2 (threonine 38 and threonine 72, respectively) abrogated the ability of each of these proteins to superactivate reporter gene expression. Phosphoamino acid analysis of radiolabeled Ets2 revealed that Ras induced normally absent threonine-specific phosphorylation of the protein. The Ras-dependent increase in threonine phosphorylation was not observed in Ets2 proteins that had the conserved threonine 72 residue mutated to alanine or serine. These data indicate that Ets1 and Ets2 are specific nuclear targets of Ras signaling events and that phosphorylation of a conserved threonine residue is a necessary molecular component of Ras-mediated activation of these transcription factors.
The mouse urokinase-type plasminogen activator (uPA) gene was used as a model macrophage colony-stimulating factor 1 (CSF-1)-inducible gene to investigate CSF-1 signalling pathways. Nuclear run-on analysis showed that induction of uPA mRNA by CSF-1 and phorbol myristate acetate (PMA) was at the transcriptional level in bone marrow-derived macrophages. CSF-1 and PMA synergized strongly in the induction of uPA mRNA, showing that at least some components of CSF-1 action are mediated independently of protein kinase C. Promoter targets of CSF-1 signalling were investigated with NIH 3T3 cells expressing the human CSF-1 receptor (c-fms). uPA mRNA was induced in these cells by treatment with CSF-1, and a PEA3/AP-1 element at -2.4 kb in the uPA promoter was involved in this response. Ets transcription factors can act through PEA3 sequences, and the involvement of Ets factors in the induction of uPA was confirmed by use of a dominant negative Ets-2 factor. Expression of the DNA binding domain of Ets-2 fused to the lacZ gene product prevented CSF-1-mediated induction of uPA mRNA in NIH 3T3 cells expressing the CSF-1 receptor. Examination of ets-2 mRNA expression in macrophages showed that it was also induced synergistically by CSF-1 and PMA. In the macrophage cell line RAW264, the uPA PEA3/AP-1 element mediated a response to both PMA and cotransfected Ets-2. uPA promoter constructs were induced 60- to 130-fold by Ets-2 expression, and the recombinant Ets-2 DNA binding domain was able to bind to the uPA PEA3/AP-1 element. This work is consistent with a proposed pathway for CSF-1 signalling involving sequential activation of fms, ras, and Ets factors.
To address the role of ras signaling in monocytic phagocytes in vivo, the expression of two dominant suppressors of in vitro ras signaling pathways, the carboxyl-terminal region of the GTPase-activating protein (GAP-C) and the DNA binding domain of the transcription factor ets-2, were targeted to this cell compartment. A 5-kb portion of the human c-fms proximal promoter was shown to direct expression of the transgenes to the monocytic lineage. As a result of the GAP-C transgene expression, ras-GTP levels were reduced in mature peritoneal macrophages by 70%. The terminal differentiation of monocytes was altered, as evidence by the accumulation of atypical monocytic cells in the blood. Mature peritoneal macrophages exhibited changes in colony-stimulating factor 1-dependent survival and structure. Further, expression of the colony-stimulating factor 1-stimulated gene urokinase plasminogen activator was inhibited in peritoneal macrophages. The results indicate that ras action is critical in monocytic cells after these cells have lost the capacity to traverse the cell cycle.
The activity of p21ras is required for the proliferative response to colony-stimulating factor 1 (CSF-1), and signals transduced by both the CSF-1 receptor (CSF-1R) and p21ras stimulate transcription from promoter elements containing overlapping binding sites for Fos/Jun- and Ets-related proteins. A sequence encoding the DNA-binding domain and nuclear localization signal of human c-ets-2, which lacked portions of the c-ets-2 gene product necessary for trans activation, was fused to the bacterial lacZ gene and expressed from an actin promoter in NIH 3T3 cells expressing either the v-ras oncogene or human CSF-1R. Nuclear expression of the Ets-LacZ protein, confirmed by histochemical staining of beta-galactosidase, inhibited the activity of ras-responsive enhancer elements and suppressed morphologic transformation by v-ras as well as CSF-1R-dependent colony formation in semisolid medium. When CSF-1R-bearing cells expressing the Ets-LacZ protein were stimulated by CSF-1, induction of c-ets-2, c-jun, and c-fos ensued, but the c-myc response was impaired. Enforced expression of the c-myc gene overrode the suppressive effect of ets-lacZ and restored the ability of these cells to form colonies in response to CSF-1. NIH 3T3 cells engineered to express a CSF-1R (Phe-809) mutant similarly cannot form CSF-1-dependent colonies in semisolid medium and exhibit an impaired c-myc response, but expression of an exogenous myc gene resensitizes these cells to CSF-1 [M. F. Roussel, J. L. Cleveland, S. A. Shurtleff, and C. J. Sherr, Nature (London) 353:361-363, 1991]. The ability of these cells to respond to CSF-1 was also rescued by enforced expression of an endogenous c-ets-2 gene. The ets family of transcription factors therefore plays a central role in integrating both CSF-1R and ras-induced mitogenic signals and in modulating the myc response to CSF-1 stimulation.
The activity of a murine VL30 transcriptional element was increased 20-fold in transient assays by coexpression of mutant ras genes. The cis element did not respond to ras in a revertant cell line that was transformation defective. Therefore, ras-dependent alterations in transcription and ras transformation are linked. Deletion analysis of the VL30 long-terminal-repeat U3 region showed that a minimal 53-base-pair segment is required in cis for oncogene activation of transcription. Gel retention assays using a probe that contained the minimal cis element revealed that a unique complex was formed with nuclear proteins prepared from transformed cells. Exonuclease III footprinting and gel retention experiments that used oligonucleotide probes and competitors indicated that two distinct nuclear factors interact with the minimal cis-responsive element. Site-directed deletion of the 5'-proximal binding site (TGACTCT) resulted in a complete loss of ras responsiveness. However, deletion of this site did not affect stimulation by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). These data are consistent with the hypothesis that ras and TPA signal transduction mechanisms for transcriptional activation are distinct.
In vitro experiments with cell extracts prepared from a mouse mammary epithelial cell line demonstrated that a cis-acting glucocorticoid response element (GRE) of the mouse mammary tumor virus represses transcription from its homologous promoter. Competition transcription experiments, in which a molar excess of a restriction fragment that contains the GRE is added to the cell-free assay, revealed that a nuclear factor mediates in trans the negative regulation of mammary tumor virus transcription in vitro. Gel retention assays indicated that a factor in the extracts specifically recognizes the GRE. One unusual result of the gel retention studies was that heating the GRE probe to 65 degrees C before addition to a binding assay increases the formation of the specific protein-DNA complex 20-fold. Exonuclease III footprinting demonstrated that the sequences recognized by the factor are identical for either untreated or heat-treated probe. The footprinting also demonstrated that this factor recognizes sequences that are distinct from those recognized by the glucocorticoid receptor. A synthetic oligonucleotide based on the sequences identified by the footprinting experiments repressed the activity of a heterologous enhancer-promoter in vivo, as assayed by transient expression assays. We propose that this negative transcription element may control the basal level of expression of some glucocorticoid-modulated genes and may explain the insensitivity of certain tumor cells to steroid hormone action.
Hormone treatment of NIH 3T3 cells that contain recombinant fusions between the mouse mammary virus long terminal repeat and the v-ras gene of Harvey murine sarcoma virus results in conditional expression of the ras p21 gene product. Levels of ras mRNA and p21 are maximal after 2 to 4 h of hormone treatment. Analysis of cellular RNA by Northern blotting and nuclease S1 protection assays indicates that the expression of two cellular RNA species increases with kinetics similar to v-ras: v-sis-related RNA and retrovirus-related VL30 RNA. Run-on transcription in isolated nuclei shows that the increase in v-sis-related RNA is not dependent on transcription and therefore must arise by a post-transcriptional mechanism. The increase in VL30 expression is a transcriptional effect. Hormone treatment of normal NIH 3T3 cells has no effect on the expression of these DNA sequences. These results suggest that v-ras stimulation of autocrine factors may play a role in transformation of cells by this gene and also suggest a reverse genetic strategy to determine the nucleic acid sequences and cellular factors involved in the regulation of gene expression that is observed.
The mouse mammary tumor virus long terminal repeat (MMTV LTR) has been introduced into cultured murine cells, using the 69% transforming fragment of bovine papilloma virus type 1 (BPV). Transformed cells contain up to 200 copies of the chimeric molecules per diploid genome. The restriction endonuclease map of the acquired recombinants, as well as the physical structure of the DNA, indicates that the LTR-BPV molecules present in these cells occur exclusively as unintegrated, extrachromosomal episome. When a 72-base pair direct repeat "enhancer" element (derived from the Harvey sarcoma retrovirus) was included in the MMTV LTR-BPV chimeric plasmids, DNA acquired through transfection, with a single exception, was integrated or rearranged or both. The transcriptional potential of the episomal MMTV promoter present in these cells was tested in two ways. First, steady-state levels of MMTV-initiated RNA were measured by quantitative S1 mapping. Second, the relative number of transcription complexes initiated in vivo was determined by using a subnuclear fraction highly enriched for MMTV-BPV minichromosomes in an in vitro transcription extension assay. Both approaches showed that the MMTV LTR present in the episomal state was capable of supporting glucocorticoid hormone-regulated transcription. We have therefore demonstrated the hormone response for the first time in a totally defined primary sequence environment. Significant differences both in the basal level of MMTV-initiated transcription and in the extent of glucocorticoid induction were observed in individual cell lines with similar episomal copy numbers. These phenotypic variations suggest that epigenetic structure is an important component of the mechanism of regulation.
A procedure for the enrichment of minichromosomes, composed of bovine papilloma virus and the long terminal repeat element of the mouse mammary tumor virus (MTV), from isolated nuclei is described. Up to 60% of the minichromosomes were extracted as nucleoprotein particles. These particles sediment in sucrose gradients as 160S complexes. Hormone-labeled glucocorticoid receptor co-purifies with these complexes in a specific fashion. Between four and six molecules of receptor are bound per minichromosome molecule. Analysis of DNase I hypersensitivity demonstrates that hypersensitive sites are preserved through the purification procedure in a manner that reflects the hormone-dependent in vivo pattern of digestion. These purified minichromosomes will allow features of chromatin structure that may be important for steroid hormone modulation of transcription to be studied in vitro without resorting to destructive nuclease digestion procedures.
In order to define the domains of the v-myb protein that are important for transactivation of gene expression, we have studied transactivation by the v-myb gene and a set of v-myb deletion mutants using transient transfection assays in NIH 3T3 cells. Analysis of the set of v-myb deletion products demonstrated that a previously unidentified region in the carboxyl-terminal portion of the protein is required for transactivation. This region lies between amino acids 295-356 with respect to the 5' end of the v-myb gene. Switching the v-myb DNA binding domain with the DNA binding domain of the rat glucocorticoid receptor (rGR) switched the cis-element requirement for v-myb action: only reports containing glucocorticoid response elements were activated by myb-rGR fusion proteins. The carboxyl terminal region essential for transactivation by the intact v-myb gene was also necessary for transactivation by the rGR-fusion gene. Carboxyl-terminal deletion mutations that encompassed the novel transactivation region were able to block wild-type v-myb transactivation when tested in transient co-expression assays. In an unexpected sidelight to our studies, we could demonstrate that the lacZ gene present in the prokaryotic vector sequences contained a DNA element that fortuitously can act as a v-myb-dependent enhancer element, and that v-myb protein can bind to this element in vitro. The lacZ enhancer contains the myb consensus DNA binding site YAAC(G/T)G.