Archaeal integrative genetic elements and their impact on genome evolution

Archaeal tyrosine recombinases

The integration of mobile genetic elements into their host chromosome influences the immediate fate of cellular organisms and gradually shapes their evolution. Site-specific recombinases catalyzing this integration have been extensively characterized both in bacteria and eukarya. More recently, a number of reports provided the in-depth characterization of archaeal tyrosine recombinases and highlighted new particular features not observed in the other two domains. In addition to being active in extreme environments, archaeal integrases catalyze reactions beyond site-specific recombination. Some of these integrases can catalyze low-sequence specificity recombination reactions with the same outcome as homologous recombination events generating deep rearrangements of their host genome. A large proportion of archaeal integrases are termed suicidal due to the presence of a specific recombination target within their own gene. The paradoxical maintenance of integrases that disrupt their gene upon integration implies novel mechanisms for their evolution. In this review, we assess the diversity of the archaeal tyrosine recombinases using a phylogenomic analysis based on an exhaustive similarity network. We outline the biochemical, ecological and evolutionary properties of these enzymes in the context of the families we identified and emphasize similarities and differences between archaeal recombinases and their bacterial and eukaryal counterparts.

Integrated mobile genetic elements in Thaumarchaeota

To explore the diversity of mobile genetic elements (MGE) associated with archaea of the phylum Thaumarchaeota, we exploited the property of most MGE to integrate into the genomes of their hosts. Integrated MGE (iMGE) were identified in 20 thaumarchaeal genomes amounting to 2 Mbp of mobile thaumarchaeal DNA. These iMGE group into five major classes: (i) proviruses, (ii) casposons, (iii) insertion sequence-like transposons, (iv) integrative-conjugative elements and (v) cryptic integrated elements. The majority of the iMGE belong to the latter category and might represent novel families of viruses or plasmids. The identified proviruses are related to tailed viruses of the order Caudovirales and to tailless icosahedral viruses with the double jelly-roll capsid proteins. The thaumarchaeal iMGE are all connected within a gene sharing network, highlighting pervasive gene exchange between MGE occupying the same ecological niche. The thaumarchaeal mobilome carries multiple auxiliary metabolic genes, including multicopper oxidases and ammonia monooxygenase subunit C (AmoC), and stress response genes, such as those for universal stress response proteins (UspA). Thus, iMGE might make important contributions to the fitness and adaptation of their hosts. We identified several iMGE carrying type I-B CRISPR-Cas systems and spacers matching other thaumarchaeal iMGE, suggesting antagonistic interactions between coexisting MGE and symbiotic relationships with the ir archaeal hosts.

Copy number variation is associated with gene expression change in archaea

Genomic instability, although frequently deleterious, is also an important mechanism for microbial adaptation to environmental change. Although widely studied in bacteria, in archaea the effect of genomic instability on organism phenotypes and fitness remains unclear. Here we use DNA segmentation methods to detect and quantify genome-wide copy number variation (CNV) in large compendia of high-throughput datasets in a model archaeal species, Halobacterium salinarum. CNV hotspots were identified throughout the genome. Some hotspots were strongly associated with changes in gene expression, suggesting a mechanism for phenotypic innovation. In contrast, CNV hotspots in other genomic loci left expression unchanged, suggesting buffering of certain phenotypes. The correspondence of CNVs with gene expression was validated with strain- and condition-matched transcriptomics and DNA quantification experiments at specific loci. Significant correlation of CNV hotspot locations with the positions of known insertion sequence (IS) elements suggested a mechanism for generating genomic instability. Given the efficient recombination capabilities in H. salinarum despite stability at the single nucleotide level, these results suggest that genomic plasticity mediated by IS element activity can provide a source of phenotypic innovation in extreme environments.

The λ Integrase Site-specific Recombination Pathway

The site-specific recombinase encoded by bacteriophage λ (Int) is responsible for integrating and excising the viral chromosome into and out of the chromosome of its Escherichia coli host. Int carries out a reaction that is highly directional, tightly regulated, and depends upon an ensemble of accessory DNA bending proteins acting on 240 bp of DNA encoding 16 protein binding sites. This additional complexity enables two pathways, integrative and excisive recombination, whose opposite, and effectively irreversible, directions are dictated by different physiological and environmental signals. Int recombinase is a heterobivalent DNA binding protein and each of the four Int protomers, within a multiprotein 400 kDa recombinogenic complex, is thought to bind and, with the aid of DNA bending proteins, bridge one arm- and one core-type DNA site. In the 12 years since the publication of the last review focused solely on the λ site-specific recombination pathway in Mobile DNA II, there has been a great deal of progress in elucidating the molecular details of this pathway. The most dramatic advances in our understanding of the reaction have been in the area of X-ray crystallography where protein-DNA structures have now been determined for of all of the DNA-protein interfaces driving the Int pathway. Building on this foundation of structures, it has been possible to derive models for the assembly of components that determine the regulatory apparatus in the P-arm, and for the overall architectures that define excisive and integrative recombinogenic complexes. The most fundamental additional mechanistic insights derive from the application of hexapeptide inhibitors and single molecule kinetics.

Identification and characterization of SNJ2, the first temperate pleolipovirus integrating into the genome of the SNJ1-lysogenic archaeal strain

Proviral regions have been identified in the genomes of many haloarchaea, but only a few archaeal halophilic temperate viruses have been studied. Here, we report a new virus, SNJ2, originating from archaeal strain Natrinema sp. J7-1. We demonstrate that this temperate virus coexists with SNJ1 virus and is dependent on SNJ1 for efficient production. Here, we show that SNJ1 is an icosahedral membrane-containing virus, whereas SNJ2 is a pleomorphic one. Instead of producing progeny virions and forming plaques, SNJ2 integrates into the host tRNA(Met) gene. The virion contains a discontinuous, circular, double-stranded DNA genome of 16 992 bp, in which both nicks and single-stranded regions are present preceded by a 'GCCCA' motif. Among 25 putative SNJ2 open reading frames (ORFs), five of them form a cluster of conserved ORFs homologous to archaeal pleolipoviruses isolated from hypersaline environments. Two structural protein encoding genes in the conserved cluster were verified in SNJ2. Furthermore, SNJ2-like proviruses containing the conserved gene cluster were identified in the chromosomes of archaea belonging to 10 different genera. Comparison of SNJ2 and these proviruses suggests that they employ a similar integration strategy into a tRNA gene.

Site-Specific Recombination by SSV2 Integrase: Substrate Requirement and Domain Functions

SSV-type integrases, encoded by fuselloviruses which infect the hyperthermophilic archaea of the Sulfolobales, are archaeal members of the tyrosine recombinase family. These integrases catalyze viral integration into and excision from a specific site on the host genome. In the present study, we have established an in vitro integration/excision assay for SSV2 integrase (Int(SSV2)). Int(SSV2) alone was able to catalyze both integration and excision reactions in vitro. A 27-bp specific DNA sequence is minimally required for the activity of the enzyme, and its flanking sequences influence the efficiency of integration by the enzyme in a sequence-nonspecific manner. The enzyme forms a tetramer through interactions in the N-terminal part (residues 1 to 80), interacts nonspecifically with DNA and performs chemical catalysis in the C-terminal part (residues 165 to 328), and appears to recognize and bind the specific site of recombination in the middle portion (residues 81 to 164). It is worth noting that an N-terminally truncated mutant of Int(SSV2) (residues 81 to 328), which corresponded to the putative product of the 3'-end sequence of the Int(SSV2) gene of the integrated SSV2 genome, was unable to form tetramers but possessed all the catalytic properties of full-length Int(SSV2) except for the slightly reduced recombination activity. Our results suggest that, unlike λ integrase, SSV-type integrases alone are capable of catalyzing viral DNA recombination with the host genome in a simple and reversible fashion.

IMPORTANCE

Archaea are host to a variety of viruses. A number of archaeal viruses are able to integrate their genome into the host genome. Many known archaeal viral integrases belong to a unique type, or the SSV type, of tyrosine recombinases. SSV-type integrases catalyze viral integration into and excision from a specific site on the host genome. However, the molecular details of the recombination process have yet to be fully understood because of the lack of an established in vitro recombination assay system. Here we report an in vitro assay for integration and excision by SSV2 integrase, a member of the SSV-type integrases. We show that SSV2 integrase alone is able to catalyze both integration and excision and reveal how different parts of the target DNA and the enzyme serve their roles in these processes. Therefore, our results provide mechanistic insights into a simple recombination process catalyzed by an archaeal integrase.

Archaeal extrachromosomal genetic elements

SUMMARY

Research on archaeal extrachromosomal genetic elements (ECEs) has progressed rapidly in the past decade. To date, over 60 archaeal viruses and 60 plasmids have been isolated. These archaeal viruses exhibit an exceptional diversity in morphology, with a wide array of shapes, such as spindles, rods, filaments, spheres, head-tails, bottles, and droplets, and some of these new viruses have been classified into one order, 10 families, and 16 genera. Investigation of model archaeal viruses has yielded important insights into mechanisms underlining various steps in the viral life cycle, including infection, DNA replication and transcription, and virion egression. Many of these mechanisms are unprecedented for any known bacterial or eukaryal viruses. Studies of plasmids isolated from different archaeal hosts have also revealed a striking diversity in gene content and innovation in replication strategies. Highly divergent replication proteins are identified in both viral and plasmid genomes. Genomic studies of archaeal ECEs have revealed a modular sequence structure in which modules of DNA sequence are exchangeable within, as well as among, plasmid families and probably also between viruses and plasmids. In particular, it has been suggested that ECE-host interactions have shaped the coevolution of ECEs and their archaeal hosts. Furthermore, archaeal hosts have developed defense systems, including the innate restriction-modification (R-M) system and the adaptive CRISPR (clustered regularly interspaced short palindromic repeats) system, to restrict invasive plasmids and viruses. Together, these interactions permit a delicate balance between ECEs and their hosts, which is vitally important for maintaining an innovative gene reservoir carried by ECEs. In conclusion, while research on archaeal ECEs has just started to unravel the molecular biology of these genetic entities and their interactions with archaeal hosts, it is expected to accelerate in the next decade.

Replication and Active Partition of Integrative and Conjugative Elements (ICEs) of the SXT/R391 Family: The Line between ICEs and Conjugative Plasmids Is Getting Thinner

Integrative and Conjugative Elements (ICEs) of the SXT/R391 family disseminate multidrug resistance among pathogenic Gammaproteobacteria such as Vibrio cholerae. SXT/R391 ICEs are mobile genetic elements that reside in the chromosome of their host and eventually self-transfer to other bacteria by conjugation. Conjugative transfer of SXT/R391 ICEs involves a transient extrachromosomal circular plasmid-like form that is thought to be the substrate for single-stranded DNA translocation to the recipient cell through the mating pore. This plasmid-like form is thought to be non-replicative and is consequently expected to be highly unstable. We report here that the ICE R391 of Providencia rettgeri is impervious to loss upon cell division. We have investigated the genetic determinants contributing to R391 stability. First, we found that a hipAB-like toxin/antitoxin system improves R391 stability as its deletion resulted in a tenfold increase of R391 loss. Because hipAB is not a conserved feature of SXT/R391 ICEs, we sought for alternative and conserved stabilization mechanisms. We found that conjugation itself does not stabilize R391 as deletion of traG, which abolishes conjugative transfer, did not influence the frequency of loss. However, deletion of either the relaxase-encoding gene traI or the origin of transfer (oriT) led to a dramatic increase of R391 loss correlated with a copy number decrease of its plasmid-like form. This observation suggests that replication initiated at oriT by TraI is essential not only for conjugative transfer but also for stabilization of SXT/R391 ICEs. Finally, we uncovered srpMRC, a conserved locus coding for two proteins distantly related to the type II (actin-type ATPase) parMRC partitioning system of plasmid R1. R391 and plasmid stabilization assays demonstrate that srpMRC is active and contributes to reducing R391 loss. While partitioning systems usually stabilizes low-copy plasmids, srpMRC is the first to be reported that stabilizes a family of ICEs.

T(lys), a newly identified Sulfolobus spindle-shaped virus 1 transcript expressed in the lysogenic state, encodes a DNA-binding protein interacting at the promoters of the early genes

While studying the gene expression of the Sulfolobus spindle-shaped virus 1 (SSV1) in Sulfolobus solfataricus lysogenic cells, a novel viral transcript (T(lys)) was identified. Transcriptional analysis revealed that T(lys) is expressed only in the absence of UV irradiation and is downregulated during the growth of the lysogenic host. The correponding gene f55 lies between two transcriptional units (T6 and T(ind)) that are upregulated upon UV irradiation. The open reading frame f55 encodes a 6.3-kDa protein which shows sequence identity with negative regulators that fold into the ribbon-helix-helix DNA-binding motif. DNA-binding assays demonstrated that the recombinant F55, purified from Escherichia coli, is indeed a putative transcription factor able to recognize site specifically target sequences in the promoters of the early induced T5, T6, and T(ind) transcripts, as well as of its own promoter. Binding sites of F55 are included within a tandem-repeated sequence overlapping the transcription start sites and/or the B recognition element of the pertinent genes. The strongest binding was observed with the promoters of T5 and T6, and an apparent cooperativity in binding was observed with the T(ind) promoter. Taking together the transcriptional analysis data and the biochemical evidences, we surmise that the protein F55 is involved in the regulation of the lysogenic state of SSV1.

Genetic studies on the virus-like regions in the genome of hyperthermophilic archaeon, Thermococcus kodakarensis

Four virus-like integrated elements (TKV1, TKV2, TKV3, and TKV4) have been found in the genome of hyperthermophilic archaeon, Thermococcus kodakarensis, but virus particle formation has not been observed in the culture of T. kodakarensis. As the result of growth property analyses, mutants lacking each of the four virus-like regions exhibited decrease in the cell concentration and/or less growth rates compared to growth of parental strain (KU216), when the T. kodakarensis strains were grown at 85 °C in nutrient-rich medium. These results indicated that the genes in virus-like regions stimulated the cell growth under the observed growth condition. As the result of transcriptome analyses, genes involved in amino acid, energy or nucleotide metabolisms, and transport systems were up- or down-regulated in the cells of mutant strains. Interestingly, a decrease in transcriptional levels of glutamine synthetase (TK1796) gene (Tk-glnA) was observed in the cells of four mutant strains. Growths of TKV1 disrupted strain and TKV4 disrupted strain have shown no difference compared with that of KU216 by the addition of glutamate or glutamine, and the result suggested that TKV1 and TKV4 contributed to supply of amino acids to the cell.

Diversity and evolution of multiple orc/cdc6-adjacent replication origins in haloarchaea

Background

While multiple replication origins have been observed in archaea, considerably less is known about their evolutionary processes. Here, we performed a comparative analysis of the predicted (proved in part) orc/cdc6-associated replication origins in 15 completely sequenced haloarchaeal genomes to investigate the diversity and evolution of replication origins in halophilic Archaea.

Results

Multiple orc/cdc6-associated replication origins were predicted in all of the analyzed haloarchaeal genomes following the identification of putative ORBs (origin recognition boxes) that are associated with orc/cdc6 genes. Five of these predicted replication origins in Haloarcula hispanica were experimentally confirmed via autonomous replication activities. Strikingly, several predicted replication origins in H. hispanica and Haloarcula marismortui are located in the distinct regions of their highly homologous chromosomes, suggesting that these replication origins might have been introduced as parts of new genomic content. A comparison of the origin-associated Orc/Cdc6 homologs and the corresponding predicted ORB elements revealed that the replication origins in a given haloarchaeon are quite diverse, while different haloarchaea can share a few conserved origins. Phylogenetic and genomic context analyses suggested that there is an original replication origin (oriC1) that was inherited from the ancestor of archaea, and several other origins were likely evolved and/or translocated within the haloarchaeal species.

Conclusion

This study provides detailed information about the diversity of multiple orc/cdc6-associated replication origins in haloarchaeal genomes, and provides novel insight into the evolution of multiple replication origins in Archaea.

Archaeal viruses--novel, diverse and enigmatic

Recent research has revealed a remarkable diversity of viruses in archaeal-rich environments where spindles, spheres, filaments and rods are common, together with other exceptional morphotypes never recorded previously. Moreover, their double-stranded DNA genomes carry very few genes exhibiting homology to those of bacterial and eukaryal viruses. Studies on viral life cycles are still at a preliminary stage but important insights are being gained especially from microarray analyses of viral transcripts for a few model virus-host systems. Recently, evidence has been presented for some exceptional archaeal-specific mechanisms for extra-cellular morphological development of virions and for their cellular extrusion. Here we summarise some of the recent developments in this rapidly developing and exciting research area.

Provirus induction in hyperthermophilic archaea: characterization of Aeropyrum pernix spindle-shaped virus 1 and Aeropyrum pernix ovoid virus 1

By in silico analysis, we have identified two putative proviruses in the genome of the hyperthermophilic archaeon Aeropyrum pernix, and under special conditions of A. pernix growth, we were able to induce their replication. Both viruses were isolated and characterized. Negatively stained virions of one virus appeared as pleomorphic spindle-shaped particles, 180 to 210 nm by 40 to 55 nm, with tails of heterogeneous lengths in the range of 0 to 300 nm. This virus was named Aeropyrum pernix spindle-shaped virus 1 (APSV1). Negatively stained virions of the other virus appeared as slightly irregular oval particles with one pointed end, while in cryo-electron micrographs, the virions had a regular oval shape and uniform size (70 by 55 nm). The virus was named Aeropyrum pernix ovoid virus 1 (APOV1). Both viruses have circular, double-stranded DNA genomes of 38,049 bp for APSV1 and 13,769 bp for APOV1. Similarities to proteins of other archaeal viruses were limited to the integrase and Dna1-like protein. We propose to classify APOV1 into the family Guttaviridae.

Discovery of permuted and recently split transfer RNAs in Archaea

Background

As in eukaryotes, precursor transfer RNAs in Archaea often contain introns that are removed in tRNA maturation. Two unrelated archaeal species display unique pre-tRNA processing complexity in the form of split tRNA genes, in which two to three segments of tRNAs are transcribed from different loci, then trans-spliced to form a mature tRNA. Another rare type of pre-tRNA, found only in eukaryotic algae, is permuted, where the 3' half is encoded upstream of the 5' half, and must be processed to be functional.

Results

Using an improved version of the gene-finding program tRNAscan-SE, comparative analyses and experimental verifications, we have now identified four novel trans-spliced tRNA genes, each in a different species of the Desulfurococcales branch of the Archaea: tRNA^Asp(GUC) in Aeropyrum pernix and Thermosphaera aggregans, and tRNA^Lys(CUU) in Staphylothermus hellenicus and Staphylothermus marinus. Each of these includes features surprisingly similar to previously studied split tRNAs, yet comparative genomic context analysis and phylogenetic distribution suggest several independent, relatively recent splitting events. Additionally, we identified the first examples of permuted tRNA genes in Archaea: tRNA^iMet(CAU) and tRNA^Tyr(GUA) in Thermofilum pendens, which appear to be permuted in the same arrangement seen previously in red alga.

Conclusions

Our findings illustrate that split tRNAs are sporadically spread across a major branch of the Archaea, and that permuted tRNAs are a new shared characteristic between archaeal and eukaryotic species. The split tRNA discoveries also provide new clues to their evolutionary history, supporting hypotheses for recent acquisition via viral or other mobile elements.

A cytidine deaminase edits C to U in transfer RNAs in Archaea

All canonical transfer RNAs (tRNAs) have a uridine at position 8, involved in maintaining tRNA tertiary structure. However, the hyperthermophilic archaeon Methanopyrus kandleri harbors 30 (out of 34) tRNA genes with cytidine at position 8. Here, we demonstrate C-to-U editing at this location in the tRNA's tertiary core, and present the crystal structure of a tRNA-specific cytidine deaminase, CDAT8, which has the cytidine deaminase domain linked to a tRNA-binding THUMP domain. CDAT8 is specific for C deamination at position 8, requires only the acceptor stem hairpin for activity, and belongs to a unique family within the "cytidine deaminase-like" superfamily. The presence of this C-to-U editing enzyme guarantees the proper folding and functionality of all M. kandleri tRNAs.

A chimeric Cre recombinase with regulated directionality

From bacterial viruses to humans, site-specific recombination and transposition are the major pathways for rearranging genomes on both long- and short-time scales. The site-specific pathways can be divided into 2 groups based on whether they are stochastic or regulated. Recombinases Cre and lambda Int are well-studied examples of each group, respectively. Both have been widely exploited as powerful and flexible tools for genetic engineering: Cre primarily in vivo and lambda Int primarily in vitro. Although Cre and Int use the same mechanism of DNA strand exchange, their respective reaction pathways are very different. Cre-mediated recombination is bidirectional, unregulated, does not require accessory proteins, and has a minimal symmetric DNA target. We show that when Cre is fused to the small N-terminal domain of Int, the resulting chimeric Cre recombines complex higher-order DNA targets comprising >200 bp encoding 16 protein-binding sites. This recombination requires the IHF protein, is unidirectional, and is regulated by the relative levels of the 3 accessory proteins, IHF, Xis, and Fis. In one direction, recombination depends on the Xis protein, and in the other direction it is inhibited by Xis. It is striking that regulated directionality and complexity can be conferred in a simple chimeric construction. We suggest that the relative ease of constructing a chimeric Cre with these properties may simulate the evolutionary interconversions responsible for the large variety of site-specific recombinases observed in Archaea, Eubacteria, and Eukarya.

A novel Sulfolobus non-conjugative extrachromosomal genetic element capable of integration into the host genome and spreading in the presence of a fusellovirus

An integrative non-conjugative extrachromosomal genetic element, denoted as pSSVi, has been isolated from a Sulfolobus solfataricus P2 strain and was characterized. This genetic element is a double-stranded DNA of 5740 bp in size and contains eight open reading frames (ORFs). It resembles members of the pRN plasmid family in genome organization but shows only weak similarity to the latter in conserved regions. pSSVi has a copG gene similar to that of a pRN plasmid, encodes a large replication protein which, unlike a typical pRN RepA, contains no polymerase/primase domain, and lacks the plrA gene. Interestingly, pSSVi encodes an SSV-type integrase which probably catalyzes the integration of its genome into a specific site (a tRNA(Arg) gene) in the S. solfataricus P2 genome. Like pSSVx, pSSVi can be packaged into a spindle-like viral particle and spread with the help of SSV1 or SSV2. In addition, both SSV1 and SSV2 appeared to replicate more efficiently in the presence of pSSVi. Given the versatile genetic abilities, pSSVi appears to be well suited for a role in horizontal gene transfer.

Genome dynamics in a natural archaeal population

Evolutionary processes that give rise to, and limit, diversification within strain populations can be deduced from the form and distribution of genomic heterogeneity. The extent of genomic change that distinguishes the acidophilic archaeon Ferroplasma acidarmanus fer1 from an environmental population of the same species from the same site, fer1(env), was determined by comparing the 1.94-megabase (Mb) genome sequence of the isolate with that reconstructed from 8 Mb of environmental sequence data. The fer1(env) composite sequence sampled approximately 92% of the isolate genome. Environmental sequence data were also analyzed to reveal genomic heterogeneity within the coexisting, coevolving fer1(env) population. Analyses revealed that transposase movement and the insertion and loss of blocks of novel genes of probable phage origin occur rapidly enough to give rise to heterogeneity in gene content within the local population. Because the environmental DNA was derived from many closely related individuals, it was possible to quantify gene sequence variability within the population. All but a few gene variants show evidence of strong purifying selection. Based on the small number of distinct sequence types and their distribution, we infer that the population is undergoing frequent genetic recombination, resulting in a mosaic genome pool that is shaped by selection. The larger genetic potential of the population relative to individuals within it and the combinatorial process that results in many closely related genome types may provide the basis for adaptation to environmental fluctuations.

Browsing repeats in genomes: Pygram and an application to non-coding region analysis

Background

A large number of studies on genome sequences have revealed the major role played by repeated sequences in the structure, function, dynamics and evolution of genomes. In-depth repeat analysis requires specialized methods, including visualization techniques, to achieve optimum exploratory power.

Results

This article presents Pygram, a new visualization application for investigating the organization of repeated sequences in complete genome sequences. The application projects data from a repeat index file on the analysed sequences, and by combining this principle with a query system, is capable of locating repeated sequences with specific properties. In short, Pygram provides an efficient, graphical browser for studying repeats. Implementation of the complete configuration is illustrated in an analysis of CRISPR structures in Archaea genomes and the detection of horizontal transfer between Archaea and Viruses.

Conclusion

By proposing a new visualization environment to analyse repeated sequences, this application aims to increase the efficiency of laboratories involved in investigating repeat organization in single genomes or across several genomes.

A spreadable, non-integrative and high copy number shuttle vector for Sulfolobus solfataricus based on the genetic element pSSVx from Sulfolobus islandicus

The pSSVx genetic element from Sulfolobus islandicus REY15/4 is a hybrid between a plasmid and a fusellovirus, able to be maintained in non-integrative form and to spread when the helper SSV2 virus is present in the cells. In this work, the satellite virus was engineered to obtain an Escherichia coli-Sulfolobus solfataricus shuttle vector for gene transfer and expression in S.solfataricus by fusing site-specifically the pSSVx chromosome with an E.coli plasmid replicon and the ampicillin resistance gene. The pSSVx-based vector was proven functional like the parental virus, namely it was able to spread efficiently through infected S.solfataricus cells. Moreover, the hybrid plasmid stably transformed S.solfataricus and propagated with no rearrangement, recombination or integration into the host chromosome. The high copy number of the artificial genetic element was found comparable with that calculated for the wild-type pSSVx in the new host cells, with no need of genetic markers for vector maintenance in the cells and for transfomant enrichment. The newly constructed vector was also shown to be an efficient cloning vehicle for the expression of passenger genes in S.solfataricus. In fact, a derivative plasmid carrying an expression cassette of the lacS gene encoding the beta-glycosidase from S.solfataricus under the control of the Sulfolobus chaperonine (thermosome tf55) heat shock promoter was also able to drive the expression of a functional enzyme. Complementation of the beta-galactosidase deficiency in a deletion mutant strain of S.solfataricus demonstrated that lacS gene was an efficient marker for selection of single transformants on solid minimal lactose medium.

Two novel conjugative plasmids from a single strain of Sulfolobus

Two conjugative plasmids (CPs) were isolated and characterized from the same 'Sulfolobus islandicus' strain, SOG2/4. The plasmids were separated from each other and transferred into Sulfolobus solfataricus. One has a high copy number and is not stable (pSOG1) whereas the other has a low copy number and is stably maintained (pSOG2). Plasmid pSOG2 is the first Sulfolobus CP found to have these characteristics. The genomes of both pSOG plasmids have been sequenced and were compared to each other and the available Sulfolobus CPs. Interestingly, apart from a very well-conserved core, 70 % of the pSOG1 and pSOG2 genomes is largely different and composed of a mixture of genes that often resemble counterparts in previously described Sulfolobus CPs. However, about 20 % of the predicted genes do not have known homologues, not even in other CPs. Unlike pSOG1, pSOG2 does not contain a gene for the highly conserved PlrA protein nor for obvious homologues of partitioning proteins. Unlike pNOB8 and pKEF9, both pSOG plasmids lack the so-called clustered regularly interspaced short palindrome repeats (CRISPRs). The sites of recombination between the two genomes can be explained by the presence of recombination motifs previously identified in other Sulfolobus CPs. Like other Sulfolobus CPs, the pSOG plasmids possess a gene encoding an integrase of the tyrosine recombinase family. This integrase probably mediates plasmid site-specific integration into the host chromosome at the highly conserved tRNA(Glu) loci.

Characterization of the Sulfolobus host-SSV2 virus interaction

The Sulfolobus spindle virus, SSV2, encodes a tyrosine integrase which furthers provirus formation in host chromosomes. Consistently with the prediction made during sequence analysis, integration was found to occur in the downstream half of the tRNA(Gly) (CCC) gene. In this paper we report the findings of a comparative study of SSV2 physiology in the natural host, Sulfolobus islandicus REY15/4, versus the foreign host, Sulfolobus solfataricus, and provide evidence of differently regulated SSV2 life cycles in the two hosts. In fact, whereas a significant induction of SSV2 replication takes place during the growth of the natural host REY15/4, the cellular content of SSV2 DNA remains fairly low throughout the incubation of the foreign host. The accumulation of episomal DNA in the former case cannot be traced to decreased packaging activity because of a simultaneous increase in the virus titre in the medium. In addition, the interaction between SSV2 and its natural host is characterized by the concurrence of host growth inhibition and the induction of viral DNA replication. When this virus-host interaction was investigated using S. islandicus REY15A, a strain which is closely related to the natural host, it was found that the SSV2 replication process was induced in the same way as in the natural host REY15/4.

Structural and genomic properties of the hyperthermophilic archaeal virus ATV with an extracellular stage of the reproductive cycle

A novel virus, ATV, of the hyperthermophilic archaeal genus Acidianus has the unique property of undergoing a major morphological development outside of, and independently of, the host cell. Virions are extruded from host cells as lemon-shaped tail-less particles, after which they develop long tails at each pointed end, at temperatures close to that of the natural habitat, 85 degrees C. The extracellularly developed tails constitute tubes, which terminate in an anchor-like structure that is not observed in the tail-less particles. A thin filament is located within the tube, which exhibits a periodic structure. Tail development produces a one half reduction in the volume of the virion, concurrent with a slight expansion of the virion surface. The circular, double-stranded DNA genome contains 62,730 bp and is exceptional for a crenarchaeal virus in that it carries four putative transposable elements as well as genes, which previously have been associated only with archaeal self-transmissable plasmids. In total, it encodes 72 predicted proteins, including 11 structural proteins with molecular masses in the range of 12 to 90 kDa. Several of the larger proteins are rich in coiled coil and/or low complexity sequence domains, which are unusual for archaea. One protein, in particular P800, resembles an intermediate filament protein in its structural properties. It is modified in the two-tailed, but not in the tail-less, virion particles and it may contribute to viral tail development. Exceptionally for a crenarchaeal virus, infection with ATV results either in viral replication and subsequent cell lysis or in conversion of the infected cell to a lysogen. The lysogenic cycle involves integration of the viral genome into the host chromosome, probably facilitated by the virus-encoded integrase and this process can be interrupted by different stress factors.

Plasmids and viruses of the thermoacidophilic crenarchaeote Sulfolobus

The crenarchaeote Sulfolobus spp. is a host for a remarkably large spectrum of viruses and plasmids. The genetic elements characterized so far indicate a large degree of novelty in terms of morphology (viruses) and in terms of genome content (plasmids and viruses). The viruses and conjugative plasmids encode a great number of conserved proteins of unknown function due to the lack of sequence similarity to functionally characterized proteins. These apparently essential proteins remain to be studied and should help to understand the physiology and genetics of the respective genetic elements as well as the host. Sulfolobus is one of the best-studied archaeons and could develop into an important model organism of the crenarchaea and the archaea.

The genome of Sulfolobus acidocaldarius, a model organism of the Crenarchaeota

Sulfolobus acidocaldarius is an aerobic thermoacidophilic crenarchaeon which grows optimally at 80 degrees C and pH 2 in terrestrial solfataric springs. Here, we describe the genome sequence of strain DSM639, which has been used for many seminal studies on archaeal and crenarchaeal biology. The circular genome carries 2,225,959 bp (37% G+C) with 2,292 predicted protein-encoding genes. Many of the smaller genes were identified for the first time on the basis of comparison of three Sulfolobus genome sequences. Of the protein-coding genes, 305 are exclusive to S. acidocaldarius and 866 are specific to the Sulfolobus genus. Moreover, 82 genes for untranslated RNAs were identified and annotated. Owing to the probable absence of active autonomous and nonautonomous mobile elements, the genome stability and organization of S. acidocaldarius differ radically from those of Sulfolobus solfataricus and Sulfolobus tokodaii. The S. acidocaldarius genome contains an integrated, and probably encaptured, pARN-type conjugative plasmid which may facilitate intercellular chromosomal gene exchange in S. acidocaldarius. Moreover, it contains genes for a characteristic restriction modification system, a UV damage excision repair system, thermopsin, and an aromatic ring dioxygenase, all of which are absent from genomes of other Sulfolobus species. However, it lacks genes for some of their sugar transporters, consistent with it growing on a more limited range of carbon sources. These results, together with the many newly identified protein-coding genes for Sulfolobus, are incorporated into a public Sulfolobus database which can be accessed at http://dac.molbio.ku.dk/dbs/Sulfolobus.

Sulfolobus tengchongensis spindle-shaped virus STSV1: virus-host interactions and genomic features

A virus infecting the hyperthermophilic archaeon Sulfolobus tengchongensis has been isolated from a field sample from Tengchong, China, and characterized. The virus, denoted STSV1 (Sulfolobus tengchongensis spindle-shaped virus 1), has the morphology of a spindle (230 by 107 nm) with a tail of variable length (68 nm on average) at one end and is the largest of the known spindle-shaped viruses. After infecting its host, the virus multiplied rapidly to high titers (>10(10) PFU/ml). Replication of the virus retarded host growth but did not cause lysis of the host cells. STSV1 did not integrate into the host chromosome and existed in a carrier state. The STSV1 DNA was modified in an unusual fashion, presumably by virally encoded modification systems. STSV1 harbors a double-stranded DNA genome of 75,294 bp, which shares no significant sequence similarity to those of fuselloviruses. The viral genome contains a total of 74 open reading frames (ORFs), among which 14 have a putative function. Five ORFs encode viral structural proteins, including a putative coat protein of high abundance. The products of the other nine ORFs are probably involved in polysaccharide biosynthesis, nucleotide metabolism, and DNA modification. The viral genome divides into two nearly equal halves of opposite gene orientation. This observation as well as a GC-skew analysis point to the presence of a putative viral origin of replication in the 1.4-kb intergenic region between ORF1 and ORF74. Both morphological and genomic features identify STSV1 as a novel virus infecting the genus Sulfolobus.

Genomic comparison of archaeal conjugative plasmids from Sulfolobus

All of the known self-transmissable plasmids of the Archaea have been found in the genus Sulfolobus. To gain more insight into archaeal conjugative processes, four newly isolated self-transmissable plasmids, pKEF9, pHVE14, pARN3 and pARN4, were sequenced and subjected to a comparative sequence analysis with two earlier sequenced plasmids, pNOB8 and pING1. The analyses revealed three conserved and functionally distinct sections in the genomes. Section A is considered to encode the main components of the conjugative apparatus, where two genes show low but significant sequence similarity to sections of genes encoding bacterial conjugative proteins. A putative origin of replication is located in section B, which is highly conserved in sequence and contains several perfect and imperfect direct and inverted repeats. Further downstream, in section C, an operon encoding six to nine smaller proteins is implicated in the initiation and regulation of replication. Each plasmid carries an integrase gene of the type that does not partition on integration, and there is strong evidence for their integration into host chromosomes, where they may facilitate intercellular exchange of chromosomal genes. Two plasmids contain hexameric short regularly spaced repeats (SRSR), which have been implicated in plasmid maintenance, and each plasmid carries multiple recombination motifs, concentrated in the variable regions, which likely provide sites for genomic rearrangements.

Novel RepA-MCM proteins encoded in plasmids pTAU4, pORA1 and pTIK4 from Sulfolobus neozealandicus

Three plasmids isolated from the crenarchaeal thermoacidophile Sulfolobus neozealandicus were characterized. Plasmids pTAU4 (7,192 bp), pORA1 (9,689 bp) and pTIK4 (13,638 bp) show unusual properties that distinguish them from previously characterized cryptic plasmids of the genus Sulfolobus. Plasmids pORA1 and pTIK4 encode RepA proteins, only the former of which carries the novel polymerase-primase domain of other known Sulfolobus plasmids. Plasmid pTAU4 encodes a mini-chromosome maintenance protein homolog and no RepA protein; the implications for DNA replication are considered. Plasmid pORA1 is the first Sulfolobus plasmid to be characterized that does not encode the otherwise highly conserved DNA-binding PlrA protein. Another encoded protein appears to be specific for the New Zealand plasmids. The three plasmids should provide useful model systems for functional studies of these important crenarchaeal proteins.

Intervening sequences of regularly spaced prokaryotic repeats derive from foreign genetic elements

Prokaryotes contain short DN repeats known as CRISPR, recognizable by the regular spacing existing between the recurring units. They represent the most widely distributed family of repeats among prokaryotic genomes suggesting a biological function. The origin of the intervening sequences, at present unknown, could provide clues about their biological activities. Here we show that CRISPR spacers derive from preexisting sequences, either chromosomal or within transmissible genetic elements such as bacteriophages and conjugative plasmids. Remarkably, these extrachromosomal elements fail to infect the specific spacer-carrier strain, implying a relationship between CRISPR and immunity against targeted DNA. Bacteriophages and conjugative plasmids are involved in prokaryotic population control, evolution, and pathogenicity. All these biological traits could be influenced by the presence of specific spacers. CRISPR loci can be visualized as mosaics of a repeated unit, separated by sequences at some time present elsewhere in the cell.

A reporter gene system for the hyperthermophilic archaeon Sulfolobus solfataricus based on a selectable and integrative shuttle vector

Sulfolobus solfataricus has developed into an important model organism for molecular and biochemical studies of hyperthermophilic archaea. Although a number of in vitro systems have been established for the organism, efficient tools for genetic manipulations have not yet been available for any hyperthermophile. In this work, we have developed a stable and selectable shuttle vector based on the virus SSV1 of Sulfolobus shibatae. We have introduced pUC18 for propagation in Escherichia coli and the genes pyrEF coding for orotidine-5'-monophosphate pyrophosphorylase and orotidine-5'-monophosphate decarboxylase of Sulfolobus solfataricus as selectable marker to complement pyrimidine auxotrophic mutants. Furthermore, the beta-galactosidase gene (lacS) was introduced into this vector as a reporter under the control of the strong and heat-inducible promoter of the Sulfolobus chaperonin (thermosome). After transformation of a S. solfataricus pyrEF/lacS double mutant, the vector was found to reside as a single-copy vector, stably integrated into the host chromosome via the site-specific recombination system of SSV1. Specific beta-galactosidase activities in transformants were found to be fourfold higher than in wild-type S. solfataricus cells, and increased to more than 10-fold after heat shock. Greatly increased levels of lacS mRNA were detected in Northern analyses, demonstrating that this reporter gene system is suitable for the study of regulated promoters in Sulfolobus and that the vector can also be used for the high-level expression of genes from hyperthermophilic archaea.

Genus-specific protein binding to the large clusters of DNA repeats (short regularly spaced repeats) present in Sulfolobus genomes

Short regularly spaced repeats (SRSRs) occur in multiple large clusters in archaeal chromosomes and as smaller clusters in some archaeal conjugative plasmids and bacterial chromosomes. The sequence, size, and spacing of the repeats are generally constant within a cluster but vary between clusters. For the crenarchaeon Sulfolobus solfataricus P2, the repeats in the genome fall mainly into two closely related sequence families that are arranged in seven clusters containing a total of 441 repeats which constitute ca. 1% of the genome. The Sulfolobus conjugative plasmid pNOB8 contains a small cluster of six repeats that are identical in sequence to one of the repeat variants in the S. solfataricus chromosome. Repeats from the pNOB8 cluster were amplified and tested for protein binding with cell extracts from S. solfataricus. A 17.5-kDa SRSR-binding protein was purified from the cell extracts and sequenced. The protein is N terminally modified and corresponds to SSO454, an open reading frame of previously unassigned function. It binds specifically to DNA fragments carrying double and single repeat sequences, binding on one side of the repeat structure, and producing an opening of the opposite side of the DNA structure. It also recognizes both main families of repeat sequences in S. solfataricus. The recombinant protein, expressed in Escherichia coli, showed the same binding properties to the SRSR repeat as the native one. The SSO454 protein exhibits a tripartite internal repeat structure which yields a good sequence match with a helix-turn-helix DNA-binding motif. Although this putative motif is shared by other archaeal proteins, orthologs of SSO454 were only detected in species within the Sulfolobus genus and in the closely related Acidianus genus. We infer that the genus-specific protein induces an opening of the structure at the center of each DNA repeat and thereby produces a binding site for another protein, possibly a more conserved one, in a process that may be essential for higher-order stucturing of the SRSR clusters.