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Hyperthermus butylicus, a hyperthermophilic neutrophile and anaerobe, is a member of the archaeal kingdom Crenarchaeota. Its genome consists of a single circular chromosome of 1,667,163 bp with a 53.7% G+C content. A total of 1672 genes were annotated, of which 1602 are protein-coding, and up to a third are specific to H. butylicus. In contrast to some other crenarchaeal genomes, a high level of GUG and UUG start codons are predicted. Two cdc6 genes are present, but neither could be linked unambiguously to an origin of replication. Many of the predicted metabolic gene products are associated with the fermentation of peptide mixtures including several peptidases with diverse specificities, and there are many encoded transporters. Most of the sulfur-reducing enzymes, hydrogenases and electron-transfer proteins were identified which are associated with energy production by reducing sulfur to H2S. Two large clusters of regularly interspaced repeats (CRISPRs) are present, one of which is associated with a crenarchaeal-type cas gene superoperon; none of the spacer sequences yielded good sequence matches with known archaeal chromosomal elements. The genome carries no detectable transposable or integrated elements, no inteins, and introns are exclusive to tRNA genes. This suggests that the genome structure is quite stable, possibly reflecting a constant, and relatively uncompetitive, natural environment.
Hyperthermus butylicus was isolated from a solfataric habitat with temperatures of up to 112 °C that lies on the sea-bed off the coast of the island of São Miguel in the Azores (Zillig et al. 1990). It grows between 80 and 108 °C with a broad temperature optimum. The organism utilizes peptide mixtures as carbon and energy sources but not amino acid mixtures, various synthetic peptides or undigested protein. It can also generate energy by reduction of elemental sulfur to yield H2S. Fermentation products include CO2, 1-butanol, acetic acid, phenylacetic acid and a trace of hydroxyphenyl acetic acid, which are produced in the presence or absence of sulfur and hydrogen (Zillig et al. 1990).
To date, few crenarchaeal genomes have been completely sequenced and analyzed. Those that are published are limited to three Sulfolobus species (family Sulfolobaceae, order Sulfolobales), A. pernix (family Desulfurococcaceae, order Desulfurococcales) and P. aerophilum (family Thermoprotaceae, order Thermoproteales) (reviewed in Klenk 2006). Thus, our genome-based knowledge of crenarchaeal biology is biased to the Sulfolobaceae. Moreover, this bias is reinforced by the fact that most of the characterized crenarchaeal extrachromosomal elements, including many cryptic and conjugative plasmids, as well as several novel viruses, are associated with this family (Zillig et al. 1998, Greve et al. 2004, Prangishvili and Garrett 2005). The H. butylicus genome presented here is the first to be reported for the family Pyrodictiaceae (order Desulfurococcales) and constitutes an important step in extending our knowledge of crenarchaeal diversity and of thermal biology because no other genome sequence is available for an organism with such a high optimal growth temperature.
The DNA for genome sequencing of H. butylicus type strain DSMZ 5456 (NCBI Taxonomy ID 54248) was taken from the original preparation (Zillig et al. 1990, 1991). The genome was cloned and mapped using a shot-gun strategy with plasmid vector pUC18 (average insert fragment size 2.5 kbp), with no additional large insert library. Template preparation for sequencing was performed with Qiagen robots (Qiagen Westburg, Germany) and ABI (Applied Biosystems, Foster City, USA) and MegaBACE 1000 Sequenators (Amersham Biotech, Amersham, U.K.). The genome was assembled from 9313 sequence reads with a mean trimmed read length of 739 nt, resulting in 4.4-fold mean sequence coverage. For gap closure and sequence editing, about 200 primer walking reactions were performed on plasmid clones. Several sequence ambiguities were resolved by generating and sequencing appropriate PCR fragments. The genome was assembled with the Phred-Phrap-Consed software package (Ewing and Green 1998, Ewing et al. 1998, Gordon et al. 1998).
Whole genome annotation and analysis was performed in the MUTAGEN system (Brügger et al. 2003). Protein coding genes were identified with the bacterial gene finder EasyGene (Larsen and Krogh 2003). Functional assignments are based on searches against GenBank (Benson et al. 2003), COG (Tatusov et al. 2001), SWISS-PROT (Boeckmann et al. 2003) and Pfam databases (Bateman et al. 2002). Transmembrane helices where predicted with TMHMM (Krogh et al. 2001) and signal peptides with SignalP (Bendtsen et al. 2004). tRNA genes were located with tRNAscan-SE (Lowe and Eddy 1997). Open reading frames (ORFs) with homologs in at least two other genomes were inferred to constitute genes and are included in the final annotation. After a round of manual annotation, remaining frameshifts were checked by generating and sequencing PCR products. Remaining frameshifts were considered to be authentic. Finally, the annotations were checked by a different annotator to validate their correctness.
The 1,667,163 bp circular genome of H. butylicus was analyzed and annotated using MUTAGEN (Brügger et al. 2003). About 1672 genes were identified, of which 1602 are protein-coding; only one of these, a CBf5 gene (Yoshinari et al. 2006), has been published previously. The number of predicted protein genes that are homologous with the other sequenced archaeal hyperthermophilic neutrophiles, A. pernix (Kawarabayasi et al. 1999) and P. aerophilum (Fitz-Gibbon et al. 2002), are illustrated (Figure 1A). These data demonstrate that each of the analyzed genomes carries a large number of genes, exclusive to that genome, in the range 41–44% for H. butylicus and A. pernix, and 62% for the larger genome of P. aerophilum, with a relatively low proportion of the genes (40, 36 and 26%, respectively) shared among all three organisms. These results serve to underline the considerable phylogenetic diversity among hyperthermophilic crenarchaea. The second plot (Figure 1B) demonstrates a much lower degree of gene homology between H. butylicus and the euryarchaeal hyperthermophile M. jannaschii (16%) and the bacterial hyperthermophile Aquifex aeolicus (6%), which extend over much larger evolutionary distances.
Although most archaeal genes are predicted to use an AUG start codon, for H. butylicus a large percentage of the predicted start codons were GUG (25%) or UUG (37%) using the Easygene prediction program (Larsen and Krogh 2003, Torarinsson et al. 2005). Moreover, similar values were obtained for the archaeal hyperthermophiles A. pernix and Methanopyrum kandleri, whereas P. aerophilum was predicted to employ 32% GUG but few UUG start codons (Torarinsson et al. 2005).
To test the reliability of these conclusions, we examined a selection of the H. butylicus protein coding genes predicted to use non AUG start codons. Of the 1602 identified genes, 981 were predicted to start with GUG or UUG codons. These were aligned with any conserved homologous genes from other species of crenarchaea or archaea predicted to use AUG as the primary start codon. A total of 191 genes yielded multiple alignments, where the homologous genes started within ± 45 bp of the start of the H. butylicus gene. Of these 191 H. butylicus genes, 42 genes exhibited possible alternative AUG start codons within ± 45 bp of the predicted start site, and 149 genes did not. These results strongly reinforce that the usage of GUG and UUG start codons is widespread in H. butylicus.
Twelve genes, which are conserved in other genomes, exhibit potential frameshifts in H. butylicus. Two of these carry “slippery” sequences, one of 5 As (Hbut_1481) and another of 6 Cs (Hbut_1362), which could facilitate the coding of functional proteins. Functional frame-shifts have also been reported for S. solfataricus P2 (Watanabe et al. 2002, Cobucci-Ponzano et al. 2003). The values were much lower than the 32 genes identified in P. aerophilum that were predicted to carry one or more frameshifts. The latter were assumed to occur mainly at homopolynucleotide sequences, and the high frequency was attributed to the lack of an efficient mismatch repair system (Fitz-Gibbon et al. 2002).
The G+C content of 53.7% is close to the original estimate (Zillig et al. 1990) and consistent with genomic values obtained for other sequenced archaeal hyperthermophilic neutrophiles, and is substantially higher than values determined for hyperthermophilic acidophiles (32–37% G+C). The high A+T values obtained for the latter may reflect the relative instability of purines and cytosines at low pH if the DNA is sometimes exposed to the extracellular environment (Lin and Forsdyke 2007).
Surfaces of hyperthermophile proteins are estimated to carry a relatively high proportion of the charged residues glutamic acid, arginine and lysine and a relatively low proportion of the non-charged polar residues, mainly glutamine (Cambillau and Claverie 2000). This pattern is consistent with the annotated genome of H. butylicus, where the percentage of codons for the charged amino acids is about 20%, compared with an average of 16% for mesophilic archaeal genomes, whereas the percentage of predicted glutamine codons at 1.9% is about half that found in the mesophilic genomes.
The genome is numbered from just upstream of the 16S + 23S rRNA genes, and the genome sequence is deposited in EMBL/GenBank under Accession Number CP000493.
No evidence was found for active, transposable, insertion sequence (IS) elements. However, two transposase gene fragments were detected, Hbut_0323 and Hbut_1361, where the latter lacks the gene start and is interrupted by a UAG stop codon. Both fragments are homologous to ISC1314 which occurs in two copies in A. pernix (Brügger et al. 2002). A search for imperfect repeat sequences using the program LUNA (K.B. unpublished) revealed no miniature inverted repeat transposable elements (MITEs), which occur in some crenarchaeal genomes where IS elements are present (Redder et al. 2001, Brügger et al. 2002). Furthermore, searches against the Sulfolobus Database (Brügger 2007) yielded no evidence for the presence of integrated crenarchaeal viruses, plasmids or other genetic elements. Nor were any att sites detected, which could have indicated earlier integration activity (She et al. 2004), although a single integrase gene is present (Hbut_1547). Thus, the genome appears to have been minimally influenced, at least in recent times, by either mobile element transposition or integration, and this inference is consistent with a fairly even G-C base pair distribution throughout the genome (data not shown).
Hyperthermus butylicus is an anaerobe and carries two genes for detoxification of O2, a superoxide reductase (Hbut_1161) and a peroxyredoxin (Hbut_0228), which remove superoxide without producing O2, thus maintaining a reduced state within the cell.
Consistent with the capacity to ferment peptides (Zillig et al. 1990), the H. butylicus genome encodes at least 23 putative peptidases that belong to diverse families and include peptidases, oligopeptidases, metallopeptidases and amino- and carboxy-peptidases as well as more specific enzymes such as a pyroglutamyl peptidase (Hbut_1614) and a glycoprotease (Hbut_0434). At least five of these carry putative N-terminal signal peptides suggesting that they are active extracellularly.
When supplied with tryptone, H. butylicus produces a number of by-products including CO2, 1-butanol, acetate, propionate, phenylacetate, hydroxyphenylacetate, acetophenone, hydroxyacetophenone and propylbenzene (Zillig et al. 1990). Some of these are likely to be products of amino acid degradation. For example, propionate can be produced from propionyl-CoA, a degradation product of valine, leucine and isoleucine, possibly by Hbut_0129, Hbut_0995 or Hbut_1337. However, genes encoding some enzymes from this degradation pathway, including enoyl hydratase, acyl-CoA dehydrogenase and a branched-chain amino acid aminotransferase, were not detected, suggesting that an alternative pathway exists in H. butylicus.
Phenylacetate is likely to be produced from phenylalanine via 2-phenylacetamid (Hbut_0228 and Hbut_0594), but since no mechanism is known for converting the fermentation product phenylacetate to hydroxyphenylacetate, the latter probably arises from tyrosine degradation, either via the pathway described above or by converting tyrosine to tyramine (Hbut_0224) and then to hydroxyphenylacetate by two reactions for which the relevant genes were not yet detected. The latter product probably cannot be metabolized further because H. butylicus lacks 4-hydroxyphenylacetate-3-hydroxylase, 4- hydroxyphenylacetate 1-hydroxylase and phenylacetate-CoA ligase, which occur in some other species of archaea.
Although 1-butanol is produced, no homolog of the bacterial acetoacetate-butyrate/acetate coenzyme A transferase was found, which is essential for 1-butanol production in Clostridium species (Toth et al. 1999). Acetate, another major by-product, is involved in cysteine biosynthesis where O-acetyl-L-serine + H2S yield L-cysteine and acetic acid may be catalyzed by Hbut_0113 or Hbut_1585. However, AMP can also be converted to ATP by splitting acetyl-CoA to CoA and acetic acid.
It is unclear whether H. butylicus produces 2-hydroxyacetophenone, 4′-hydroxyacetophenone or both (Zillig et al. 1990). Acetophenone and 2-hydroxyacetophenone are products of ethylbenzene degradation, with the latter produced from the former. A range of enzymes are candidates for the production of acetophenone from 1-phenylethanol. 4′-hydroxyacetophenone, however, may be produced from 1-(4′-hydroxyphenyl) ethanol.
The genome encodes a wide range of transporter proteins. Thirty-seven genes encode predicted ABC-type transporters. Moreover, five gene products were implicated in cation transport and three in anion transport. In addition, six genes encoded proteins implicated in branched chain amino acid transport.
We infer that the cell appendages implicated in cell motility (Zillig et al. 1990) were not flagella because homologs of genes encoding archaeal flagellin (FlaB), crucial for flagella formation, as well as other flagella-associated genes, are absent (Ng et al. 2006, S. Gribaldo, Institute Pasteur, Paris, pers. comm.).
Some archaeal enzymes involved in metabolizing and transporting sulfur products have been characterized experimentally. In particular, functional protein complexes from both Pyrodictium abyssi (Dirmeier et al. 1998) and Acidianus ambivalens (Laska et al. 2003), which are membrane-bound and contain multiple subunits, were shown to catalyze sulfur reduction to H2S. The protein complexes are associated with sulfur reductase, hydrogenase and electron transfer activities, and, at least for A. ambivalens, the relevant genes are clustered in operons. Sulfur reduction to H2S is an important energy generating process in H. butylicus (Zillig et al. 1990), and homologs of many of these characterized genes are clustered in operons in the genome. They include those for sulfur-reducing enzymes sreA (Hbut_0373, Hbut_1051), sreB (Hbut_0372, Hbut_1052) and sreD (Hbut_0757) and an operon containing sdhE (Hbut_0155), sdhF (Hbut_0156) and sdhB (Hbut_0157). Genes encoding hydrogenases, and other proteins considered necessary for the function of the sulfur-reducing complex, include hypC (Hbut_0754), hypD (Hbut_0755) and an operon carrying hydL (Hbut_1368), isp2 (Hbut_1369), isp1 (Hbut_1370), hydS (Hbut_1371) and hoxM (Hbut_1372), whereas tatC (Hbut_1508) and tatA (Hbut_1509) are implicated in transport (Laska et al. 2003, Kletzin 2006).
Archaea often carry multiple cdc6 genes, and for Pyrococcus and Sulfolobus species, some of these have been shown experimentally to be located close to chromosomal replication origins (Matsunaga et al. 2001, Lundgren et al. 2004, Robinson et al. 2004). Hyperthermus butylicus has two genes, cdc6-1 (Hbut_0595) and cdc6-2 (Hbut_0909), where the former is preceded by 1700 bp lacking ORFs, whereas the latter lies within an operon. Generation of Z-curve and G+C-skew plots (Zhang and Zhang 2005) yielded a few peaks and troughs that could represent replication origin(s). Although cdc6-2 gives the best match at around position 0.9 Mb in the G+C skew plot, neither gene coincides exactly with a peak or trough (Figure 2). Moreover, no ORB motifs characteristic of archaeal chromosomal origins (Robinson et al. 2004) were detected adjoining the cdc6 genes (within 2 kb on either side), although this could reflect a low level of sequence conservation of the motifs with those of A. pernix and other crenarchaea. The normal complement of genes associated with the crenarchaeal replication apparatus were present (Marsh and Bell 2006). Two chromatin/RNA binding Alba homologs are encoded (Hbut_0909 and Hbut_0977).
The genome contains many of the previously characterized archaeal DNA repair enzymes, including some involved in double-strand-break repair, direct repair, base excision repair and nucleotide excision repair (White 2006). For direct repair, 6-O-methylguanineDNA methyltransferase is encoded, and for the base excision pathway, several proteins are encoded. Moreover, consistent with the sea bed environment from which H. butylicus was isolated (Zillig et al. 1990), and in contrast to some other species of archaea (e.g. Baliga et al. 2004, Chen et al. 2005),no genes were detected for extra UV excision repair pathways. Moreover, no gene was found for a deoxyribodipyrimidine photolyase (COG0415) which could directly repair pyrimidine dimers generated by visible light, nor for 8-oxoguanine DNA glycosylase, which repairs the lesion of 8-oxoguanine (oxoG) caused by ionizing radiation or oxidizing agents (Gogos and Clarke 1999).
Furthermore, no H. butylicus gene was detected for an error-prone DNA polymerase which can accommodate a variety of modified templates and achieves trans-lesion synthesis by different mechanisms (Yang 2005). As for most sequenced archaeal genomes, genes encoding the eukaryal/bacterial-type mismatch repair proteins were not observed in the H. butylicus genome. Although a common DNA repair photolyase (Hbut_1189, COG 1533) and two RecB-like exonucleases (Hbut_0039, Hbut_0849) are encoded, their precise roles in DNA repair remain to be determined.
The total complement of crenarchaeal RNA polymerase subunits is encoded (Zillig et al. 1990), with a single rpoB gene as occurs in almost all crenarchaeal genomes (She et al. 2001). Genes for the characteristic archaeal transcription factors are also present, with a single gene for the TBP protein (Hbut_0934) and two genes for TFIIB (Hbut_0054 and Hbut_0458). For the translational apparatus, single copies of the coupled 16S and 23S rRNA genes and a single, uncoupled, 5S rRNA gene are present as well as a single 7S RNA gene and the usual complement of crenarchaeal ribosomal protein genes (26 small subunit and 38 large subunit). The ribosomal factors involved in initiation, elongation and termination of protein synthesis are also typically crenarchaeal (Londei 2006).
Most genes (74%) are preceded by Shine-Dalgarno (S-D) sequence motifs and, for many of these, predicted TATA-like motifs occur at least 25 bp further upstream. Thus, H. butylicus is unlikely to produce large numbers of leaderless transcripts as was predicted to occur for some other crenarchaea (Torarinsson et al. 2005). As shown earlier, archaeal S-D motifs are more variable than those of bacteria, and their sequences depend on the actual length of the conserved 3′-terminal sequence of 16S rRNA (Torarinsson et al. 2005). Exceptionally, H. butylicus, together with A. pernix and M. kandleri, shows a high incidence of predicted GGGG S-D motifs (Torarinsson et al. 2005).
The genome of H. butylicus contains 46 tRNA genes carrying 43 different anti-codons coding for 20 amino acids. As for most archaeal genomes, three genes are present for tRNAMet(CAT), which differ in sequence with one containing an intron located at the anti-codon loop. One of these is classified as an initiator tRNA (Hbut_1108) on the basis of sequence comparison with an experimentally characterized archaeal initiator tRNA (Kuchino et al. 1982). As for other crenarchaeal genomes, a selenocysteine incorporation system is lacking, but a selenocycteine lyase (Hbut_0769) is encoded for salvaging cysteine.
Genes for several putative RNA modifying enzymes were identified, consistent with a relatively large modification of the archaeal hyperthermophile RNAs (Rozenski et al. 1999). Searches for other untranslated RNAs, using experimentally determined sequences from other crenarchaea, failed to reveal unambiguous RNA homologs in H. butylicus. We infer, therefore, that sequence conservation is too low and that these RNAs should be identified experimentally.
Although no unambiguous gene matches were found for restriction enzymes, which may reflect the limited sequence conservation of this enzyme family, a putative DNA-modifying C-5 cytosine methylase is encoded (Hbut_1135). Moreover, H. butylicus contains two large clusters of short regular spaced repeats (SRSR/CRISPR) positioned 66.5 kb apart and containing 46 and 47 repeat-spacer units (Figure 2), and encodes a homolog of the protein (Hbut_0986) that interacts specifically with the repeat sequence in the S. solfataricus P2 chromosome and Sulfolobus plasmid pNOB8 (Peng et al. 2003). The spacer sequences are considered to derive from extrachromosomal elements and, in particular, from viruses (Mojica et al. 2005), and their spacer transcripts (Tang et al. 2002, 2005) may inactivate viral or plasmid propagation by acting at the level of mRNA or DNA (Lillestøl et al. 2006, Makarova et al. 2006). No matches were observed between the spacer sequences of H. butylicus and any known archaeal viruses and plasmids, but this negative result may reflect the unavailability of extrachromosomal genetic elements which can propagate in H. butylicus or in related organisms.
A single superoperon containing a set of cas and csa genes adjoins one of the repeat clusters. The encoded proteins are considered to be involved in the development and activity of the repeat-clusters (Jansen et al. 2002, Lillestøl et al. 2006, Makarova et al. 2006). The gene order is crenarchaea-specific (Lillestøl et al. 2006), except that the cas6 gene, which is generally located most distantly from the repeat-cluster, is absent. These genes are predicted to encode diverse nucleases cas1 (Hbut_0652), cas4 (Hbut_0649), csa2 (Hbut_0644), a DNA helicase cas3 (Hbut_0640) and an RNA binding protein cas5 (Hbut_0643) (Makarova et al. 2002, Haft et al. 2005, Makarova et al. 2006).
Given the apparent stability of the H. butylicus genome, which may be partly due to an efficient restriction-modification system, as has been argued for the similarly stable genome of S. acidocaldarius (Chen et al. 2005), the large number of repeat-spacer units in the clusters is difficult to explain. The current hypothesis for the clusters (Mojica et al. 2005) suggests that the organism has been regularly invaded by extrachromosomal elements. If true, then they have, surprisingly, left no detectable traces in the genome other than the spacers. One possible explanation is that the large archaeal clusters are multifunctional and have other important activities which the smaller, non ubiquitous, bacterial clusters lack.
Introns occur within 10 of the 46 tRNA genes. They are all archaeal-type introns and are located +1 bp after the 3′-end of the anticodon, as occurs for many archaeal tRNA introns and for all eukaryal tRNA introns (Marck and Grosjean 2003). The origin of the archaeal tRNA introns is still unclear. In archaea, rRNA intron mobility is generally facilitated by a homing endonuclease (Aagaard et al. 1995), but none appears to be encoded in the H. butylicus genome. Another hypothesis is that they arise by fusion of fragmented tRNA genes (Randau et al. 2005). A comparison of the tRNA-containing introns of H. butylicus, A. pernix and P. aerophilum (Table 1) reveals some conservation of tRNA-containing introns between H. butylicus and A. pernix (although there is no general conservation of sequence) but only three conserved tRNAs between these two organisms and P. aerophilum (Met (CAT), Pro (GGG) and Tyr (GTA)). On balance, it suggests, as was concluded from a study of the three Sulfolobus genomes (Brügger et al. 2006), that the introns can be mobilized, but presumably only when a homing endonuclease is present as it is in P. aerophilum (Burggraf et al. 1993).
No introns are present in the rRNA genes although they occur widely in other crenarchaea. To date, only one archaeal intron has been demonstrated to occur within a protein coding sequence (Watanabe et al. 2002) although others have been predicted (Brügger et al. 2006). The identified intron is located in homologs of the eukaryal Cbf5 protein gene which is a subunit of the small nucleolar RNA-protein complex. Its excision from mRNAs in A. pernix, S. solfataricus and S. tokodaii removes a frameshift from the coding region (Watanabe et al. 2002). However, the H. butylicus Cbf5 gene (Hbut_1329) lacks this intron, and this correlates with the recent study showing that the introns are confined to all studied members of the crenarchaeal families Sulfolobaceae and Desulfurococcaceae (Yoshinari et al. 2006).
Two components of the intron splicing enzyme are encoded, a catalytic subunit (Hbut_0054) and a structural subunit (Hbut_0955), which together generate the oligomeric structure for the archaeal intron-splicing enzyme characterized for Sulfolobus species (Tocchini-Valentini et al. 2005, Yoshinari et al. 2005). The catalytic subunit is conserved in size and sequence for the crenarchaea, but the structural subunit is more variable. Hyperthermus butylicus encodes the larger form (182 aa), whereas some crenarchaea, including S. tokodaii and S. acidocaldarius, encode a smaller protein (~92 aa) homologous to the C-terminal half of the larger protein (Brügger 2007).
Searches against the intein database were negative (Perler 2002). Moreover, an A. pernix ORF APE0745, carrying a predicted 468 aa intein encoding a homing endonuclease was absent from the H. butylicus homologs (Hbut_0418, Hbut_0633 and Hbut_0856). We infer that H. butylicus lacks inteins as has been predicted for some other crenarchaea (e.g., Chen et al. 2005).
The phylogenetic tree presented in Figure 3 was derived using 16S rRNA sequences. It illustrates the three major orders of the Crenarchaeota Desulfurococcales, Sulfolobales and Thermoproteales, for which several organisms have been purified and characterized (Stetter 2006). Included in the tree are those organisms (in blue) for which full genomic sequences have been published and those (in red) whose sequences are in progress or unpublished (Klenk 2006). Possibilities for comparative genomic studies of the crenarchaea are still limited because only six crenarchaeal genomes are available, three of which are from the Sulfolobus genus. As indicated in the Figure, the present genome, together with those which should soon be available, will give a strong basis for a comprehensive genomic comparison of the three best characterised orders of the kingdom Crenarchaeota.
Genome sequencing was supported by EU Cell Factory grant No. QLK3-CT-2000-00649, and further grants were received for an Archaea Centre supported by the Danish Natural Science Research Council. We thank Bettina Haberl (Epidauros Biotechnologie AG) and Hien Phan for help with DNA sequencing.