Genome evolution at the genus level: comparison of three complete genomes of hyperthermophilic archaea

Pan-genome study of Thermococcales reveals extensive genetic diversity and genetic evidence of thermophilic adaption

Thermococcales has a strong adaptability to extreme environments, which is of profound interest in explaining how complex life forms emerge on earth. However, their gene composition, thermal stability and evolution in hyperthermal environments are still little known. Here, we characterized the pan-genome architecture of 30 Thermococcales species to gain insight into their genetic properties, evolutionary patterns and specific metabolisms adapted to niches. We revealed an open pan-genome of Thermococcales comprising 6070 gene families that tend to increase with the availability of additional genomes. The genome contents of Thermococcales were flexible, with a series of genes experienced gene duplication, progressive divergence, or gene gain and loss events exhibiting distinct functional features. These archaea had concise types of heat shock proteins, such as HSP20, HSP60 and prefoldin, which were constrained by strong purifying selection that governed their conservative evolution. Furthermore, purifying selection forced genes involved in enzyme, motility, secretion system, defence system and chaperones to differ in functional constraints and their disparity in the rate of evolution may be related to adaptation to specific niche. These results deepened our understanding of genetic diversity and adaptation patterns of Thermococcales, and provided valuable research models for studying the metabolic traits of early life forms.

Biomass-degrading glycoside hydrolases of archaeal origin

During the last decades, the impact of hyperthermophiles and their enzymes has been intensively investigated for implementation in various high-temperature biotechnological processes. Biocatalysts of hyperthermophiles have proven to show extremely high thermo-activities and thermo-stabilities and are identified as suitable candidates for numerous industrial processes with harsh conditions, including the process of an efficient plant biomass pretreatment and conversion. Already-characterized archaea-originated glycoside hydrolases (GHs) have shown highly impressive features and numerous enzyme characterizations indicated that these biocatalysts show maximum activities at a higher temperature range compared to bacterial ones. However, compared to bacterial biomass-degrading enzymes, the number of characterized archaeal ones remains low. To discover new promising archaeal GH candidates, it is necessary to study in detail the microbiology and enzymology of extremely high-temperature habitats, ranging from terrestrial to marine hydrothermal systems. State-of-the art technologies such as sequencing of genomes and metagenomes and automated binning of genomes out of metagenomes, combined with classical microbiological culture-dependent approaches, have been successfully performed to detect novel promising biomass-degrading hyperthermozymes. In this review, we will focus on the detection, characterization and similarities of archaeal GHs and their unique characteristics. The potential of hyperthermozymes and their impact on high-temperature industrial applications have not yet been exhausted.

Transcriptional Regulation in Archaea: From Individual Genes to Global Regulatory Networks

Archaea are major contributors to biogeochemical cycles, possess unique metabolic capabilities, and resist extreme stress. To regulate the expression of genes encoding these unique programs, archaeal cells use gene regulatory networks (GRNs) composed of transcription factor proteins and their target genes. Recent developments in genetics, genomics, and computational methods used with archaeal model organisms have enabled the mapping and prediction of global GRN structures. Experimental tests of these predictions have revealed the dynamical function of GRNs in response to environmental variation. Here, we review recent progress made in this area, from investigating the mechanisms of transcriptional regulation of individual genes to small-scale subnetworks and genome-wide global networks. At each level, archaeal GRNs consist of a hybrid of bacterial, eukaryotic, and uniquely archaeal mechanisms. We discuss this theme from the perspective of the role of individual transcription factors in genome-wide regulation, how these proteins interact to compile GRN topological structures, and how these topologies lead to emergent, high-level GRN functions. We conclude by discussing how systems biology approaches are a fruitful avenue for addressing remaining challenges, such as discovering gene function and the evolution of GRNs.

Biogeography and evolution of Thermococcus isolates from hydrothermal vent systems of the Pacific

Thermococcus is a genus of hyperthermophilic archaea that is ubiquitous in marine hydrothermal environments growing in anaerobic subsurface habitats but able to survive in cold oxygenated seawater. DNA analyses of Thermococcus isolates were applied to determine the relationship between geographic distribution and relatedness focusing primarily on isolates from the Juan de Fuca Ridge and South East Pacific Rise. Amplified fragment length polymorphism (AFLP) analysis and multilocus sequence typing (MLST) were used to resolve genomic differences in 90 isolates of Thermococcus, making biogeographic patterns and evolutionary relationships apparent. Isolates were differentiated into regionally endemic populations however there was also evidence in some lineages of cosmopolitan distribution. The biodiversity identified in Thermococcus isolates and presence of distinct lineages within the same vent site suggests the utilization of varying ecological niches in this genus. In addition to resolving biogeographic patterns in Thermococcus, this study has raised new questions about the closely related Pyrococcus genus. The phylogenetic placement of Pyrococcus type strains shows the close relationship between Thermococcus and Pyrococcus and the unresolved divergence of these two genera.

Recombinant expression library of Pyrococcus furiosus constructed by high-throughput cloning: a useful tool for functional and structural genomics

Hyperthermophile Pyrococcus furiosus grows optimally near 100°C and is an important resource of many industrial and molecular biological enzymes. To study the structure and function of P. furiosus proteins at whole genome level, we constructed expression plasmids of each P. furiosus gene using a ligase-independent cloning method, which was based on amplifying target gene and vector by PCR using phosphorothioate-modified primers and digesting PCR products by λ exonuclease. Our cloning method had a positive clone percentage of ≥ 80% in 96-well plate cloning format. Small-scale expression experiment showed that 55 out of 80 genes were efficiently expressed in Escherichia coli Strain Rosetta 2(DE3)pLysS. In summary, this recombinant expression library of P. furiosus provides a platform for functional and structural studies, as well as developing novel industrial enzymes. Our cloning scheme is adaptable to constructing recombinant expression library of other sequenced organisms.

Structural basis of substrate selectivity of E. coli prolidase

Prolidases, metalloproteases that catalyze the cleavage of Xaa-Pro dipeptides, are conserved enzymes found in prokaryotes and eukaryotes. In humans, prolidase is crucial for the recycling of collagen. To further characterize the essential elements of this enzyme, we utilized the Escherichia coli prolidase, PepQ, which shares striking similarity with eukaryotic prolidases. Through structural and bioinformatic insights, we have extended previous characterizations of the prolidase active site, uncovering a key component for substrate specificity. Here we report the structure of E. coli PepQ, solved at 2.0 Å resolution. The structure shows an antiparallel, dimeric protein, with each subunit containing N-terminal and C-terminal domains. The C-terminal domain is formed by the pita-bread fold typical for this family of metalloproteases, with two Mg(II) ions coordinated by five amino-acid ligands. Comparison of the E. coli PepQ structure and sequence with homologous structures and sequences from a diversity of organisms reveals distinctions between prolidases from Gram-positive eubacteria and archaea, and those from Gram-negative eubacteria, including the presence of loop regions in the E. coli protein that are conserved in eukaryotes. One such loop contains a completely conserved arginine near the catalytic site. This conserved arginine is predicted by docking simulations to interact with the C-terminus of the substrate dipeptide. Kinetic analysis using both a charge-neutralized substrate and a charge-reversed variant of PepQ support this conclusion, and allow for the designation of a new role for this key region of the enzyme active site.

Genome sequencing of a genetically tractable Pyrococcus furiosus strain reveals a highly dynamic genome

The model archaeon Pyrococcus furiosus grows optimally near 100°C on carbohydrates and peptides. Its genome sequence (NCBI) was determined 12 years ago. A genetically tractable strain, COM1, was very recently reported, and here we describe its genome sequence. Of 1,909,827 bp in size, it is 1,571 bp longer (0.1%) than the reference NCBI sequence. The COM1 genome contains numerous chromosomal rearrangements, deletions, and single base changes. COM1 also has 45 full or partial insertion sequences (ISs) compared to 35 in the reference NCBI strain, and these have resulted in the direct deletion or insertional inactivation of 13 genes. Another seven genes were affected by chromosomal deletions and are predicted to be nonfunctional. In addition, the amino acid sequences of another 102 of the 2,134 predicted gene products are different in COM1. These changes potentially impact various cellular functions, including carbohydrate, peptide, and nucleotide metabolism; DNA repair; CRISPR-associated defense; transcriptional regulation; membrane transport; and growth at 72°C. For example, the IS-mediated inactivation of riboflavin synthase in COM1 resulted in a riboflavin requirement for growth. Nevertheless, COM1 grew on cellobiose, malto-oligosaccharides, and peptides in complex and minimal media at 98 and 72°C to the same extent as did both its parent strain and a new culture collection strain (DSMZ 3638). This was in spite of COM1 lacking several metabolic enzymes, including nonphosphorylating glyceraldehyde-3-phosphate dehydrogenase and beta-glucosidase. The P. furiosus genome is therefore of high plasticity, and the availability of the COM1 sequence will be critical for the future studies of this model hyperthermophile.

Re-annotation of two hyperthermophilic archaea Pyrococcus abyssi GE5 and Pyrococcus furiosus DSM 3638

Pyrococcus abyssi GE5 (P. aby) and Pyrococcus furiosus DSM 3638 (P. fur) are two model hyperthermophilic archaea. However, their annotations in public databases are unsatisfactory. In this article, the two genomes were re-annotated according to the following steps. (i) All "hypothetical genes" in the original annotation were re-identified based on the Z-curve method, and some of them were recognized as non-coding open reading frames (ORFs). Evidence showed that the recognized non-coding ORFs were highly unlikely to encode proteins. (ii) The translation initiation sites (TISs) of all the annotated genes were re-located, and more than 10% of the TISs were shifted to 5'-upstream or 3'-downstream regions. (iii) The functions of the refined "hypothetical genes" were predicted using sequence alignment tools, more than 200 originally annotated "hypothetical genes" in either of the two hyperthermophiles were assigned functions. A large number of these functions have reference support or experimentally characterized homologues. All the refined information will serve as a valuable resource for research on P. aby and P. fur, which may be helpful in the exploration of thermal adaptation mechanisms. The complete re-annotation files of P. aby and P. fur are available at http://211.69.128.148/download/ .

Archaeal RNA polymerase and transcription regulation

To elucidate the mechanism of transcription by cellular RNA polymerases (RNAPs), high-resolution X-ray crystal structures together with structure-guided biochemical, biophysical, and genetics studies are essential. The recently solved X-ray crystal structures of archaeal RNAP allow a structural comparison of the transcription machinery among all three domains of life. The archaea were once thought of closely related to bacteria, but they are now considered to be more closely related to the eukaryote at the molecular level than bacteria. According to these structures, the archaeal transcription apparatus, which includes RNAP and general transcription factors (GTFs), is similar to the eukaryotic transcription machinery. Yet, the transcription regulators, activators and repressors, encoded by archaeal genomes are closely related to bacterial factors. Therefore, archaeal transcription appears to possess an intriguing hybrid of eukaryotic-type transcription apparatus and bacterial-like regulatory mechanisms. Elucidating the transcription mechanism in archaea, which possesses a combination of bacterial and eukaryotic transcription mechanisms that are commonly regarded as separate and mutually exclusive, can provide data that will bring basic transcription mechanisms across all life forms.

Comparative genomics reveals birth and death of fragile regions in mammalian evolution

Background

An important question in genome evolution is whether there exist fragile regions (rearrangement hotspots) where chromosomal rearrangements are happening over and over again. Although nearly all recent studies supported the existence of fragile regions in mammalian genomes, the most comprehensive phylogenomic study of mammals raised some doubts about their existence.

Results

Here we demonstrate that fragile regions are subject to a birth and death process, implying that fragility has a limited evolutionary lifespan.

Conclusions

This finding implies that fragile regions migrate to different locations in different mammals, explaining why there exist only a few chromosomal breakpoints shared between different lineages. The birth and death of fragile regions as a phenomenon reinforces the hypothesis that rearrangements are promoted by matching segmental duplications and suggests putative locations of the currently active fragile regions in the human genome.

Molecular evolution of the hyperthermophilic archaea of the Pyrococcus genus: analysis of adaptation to different environmental conditions

Background

Prokaryotic microorganisms are able to survive and proliferate in severe environmental conditions. The increasing number of complete sequences of prokaryotic genomes has provided the basis for studying the molecular mechanisms of their adaptation at the genomic level. We apply here a computer-based approach to compare the genomes and proteomes from P. furiosus, P. horikoshii, and P. abyssi to identify features of their molecular evolution related to adaptation strategy to diverse environmental conditions.

Results

Phylogenetic analysis of rRNA genes from 26 Pyrococcus strains suggested that the divergence of P. furiosus, P. horikoshii and P. abyssi might have occurred from ancestral deep-sea organisms. It was demonstrated that the function of genes that have been subject to positive Darwinian selection is closely related to abiotic and biotic conditions to which archaea managed to become adapted. Divergence of the P. furiosus archaea might have been due to loss of some genes involved in cell motility or signal transduction, and/or to evolution under positive selection of the genes for translation machinery. In the course of P. horikoshii divergence, positive selection was found to operate mainly on the transcription machinery; divergence of P. abyssi was related with positive selection for the genes mainly involved in inorganic ion transport. Analysis of radical amino acid replacement rate in evolving P. furiosus, P. horikoshii and P. abyssi showed that the fixation rate was higher for radical substitutions relative to the volume of amino acid side-chain.

Conclusions

The current results give due credit to the important role of hydrostatic pressure as a cause of variability in the P. furiosus, P. horikoshii and P. abyssi genomes evolving in different habitats. Nevertheless, adaptation to pressure does not appear to be the sole factor ensuring adaptation to environment. For example, at the stage of the divergence of P. horikoshii and P. abyssi, an essential evolutionary role may be assigned to changes in the trophic chain, namely, acquisition of a consumer status at a high (P. horikoshii) or low level (P. abyssi).

Characterization of two proline dipeptidases (prolidases) from the hyperthermophilic archaeon Pyrococcus horikoshii

Prolidases hydrolyze the unique bond between X-Pro dipeptides and can also cleave the P-F and P-O bonds found in organophosphorus compounds, including the nerve agents, soman and sarin. The advantages of using hyperthermophilic enzymes in biodetoxification strategies are based on their enzyme stability and efficiency. Therefore, it is advantageous to examine new thermostable prolidases for potential use in biotechnological applications. Two thermostable prolidase homologs, PH1149 and PH0974, were identified in the genome of Pyrococcus horikoshii based on their sequences having conserved metal binding and catalytic amino acid residues that are present in other known prolidases, such as the previously characterized Pyrococcus furiosus prolidase. These P. horikoshii prolidases were expressed recombinantly in the Escherichia coli strain BL21 (lambdaDE3), and both were shown to function as proline dipeptidases. Biochemical characterization of these prolidases shows they have higher catalytic activities over a broader pH range, higher affinity for metal and are more stable compared to P. furiosus prolidase. This study has important implications for the potential use of these enzymes in biotechnological applications and provides further information on the functional traits of hyperthermophilic proteins, specifically metalloenzymes.

Protein splicing of the three Pyrococcus abyssi ribonucleotide reductase inteins

An intein is a polypeptide that interrupts the functional domains of a protein, called the exteins. The intein can facilitate its own excision from the exteins, concomitant with the ligation of the exteins, in a process called protein splicing. The alpha subunit of the ribonucleotide reductase of the extreme thermophile Pyrococcus abyssi is interrupted by three inteins in separate insertion sites. Each intein can facilitate protein splicing when over-expressed in Escherichia coli, with affinity domains serving as the exteins. The influence of the N-terminal flanking residue on the efficiency of splicing is specific to each intein. Each intein has a different downstream nucleophilic residue, and cannot tolerate substitution to a residue of lesser or equal nucleophilicity. The influence of the conserved penultimate His also differs between the inteins.

Genome analysis and genome-wide proteomics of Thermococcus gammatolerans, the most radioresistant organism known amongst the Archaea

The genome sequence of Thermococcus gammatolerans, a radioresistant archaeon, is described; a proteomic analysis reveals that radioresistance may be due to unknown DNA repair enzymes.

Background

Thermococcus gammatolerans was isolated from samples collected from hydrothermal chimneys. It is one of the most radioresistant organisms known amongst the Archaea. We report the determination and annotation of its complete genome sequence, its comparison with other Thermococcales genomes, and a proteomic analysis.

Results

T. gammatolerans has a circular chromosome of 2.045 Mbp without any extra-chromosomal elements, coding for 2,157 proteins. A thorough comparative genomics analysis revealed important but unsuspected genome plasticity differences between sequenced Thermococcus and Pyrococcus species that could not be attributed to the presence of specific mobile elements. Two virus-related regions, tgv1 and tgv2, are the only mobile elements identified in this genome. A proteogenome analysis was performed by a shotgun liquid chromatography-tandem mass spectrometry approach, allowing the identification of 10,931 unique peptides corresponding to 951 proteins. This information concurrently validates the accuracy of the genome annotation. Semi-quantification of proteins by spectral count was done on exponential- and stationary-phase cells. Insights into general catabolism, hydrogenase complexes, detoxification systems, and the DNA repair toolbox of this archaeon are revealed through this genome and proteome analysis.

Conclusions

This work is the first archaeal proteome investigation done at the stage of primary genome annotation. This archaeon is shown to use a large variety of metabolic pathways even under a rich medium growth condition. This proteogenomic study also indicates that the high radiotolerance of T. gammatolerans is probably due to proteins that remain to be characterized rather than a larger arsenal of known DNA repair enzymes.

The tree of genomes: an empirical comparison of genome-phylogeny reconstruction methods

BACKGROUND

In the past decade or more, the emphasis for reconstructing species phylogenies has moved from the analysis of a single gene to the analysis of multiple genes and even completed genomes. The simplest method of scaling up is to use familiar analysis methods on a larger scale and this is the most popular approach. However, duplications and losses of genes along with horizontal gene transfer (HGT) can lead to a situation where there is only an indirect relationship between gene and genome phylogenies. In this study we examine five widely-used approaches and their variants to see if indeed they are more-or-less saying the same thing. In particular, we focus on Conditioned Reconstruction as it is a method that is designed to work well even if HGT is present.

RESULTS

We confirm a previous suggestion that this method has a systematic bias. We show that no two methods produce the same results and most current methods of inferring genome phylogenies produce results that are significantly different to other methods.

CONCLUSION

We conclude that genome phylogenies need to be interpreted differently, depending on the method used to construct them.

Extremely thermophilic microorganisms for biomass conversion: status and prospects

Many microorganisms that grow at elevated temperatures are able to utilize a variety of carbohydrates pertinent to the conversion of lignocellulosic biomass to bioenergy. The range of substrates utilized depends on growth temperature optimum and biotope. Hyperthermophilic marine archaea (T(opt)>or=80 degrees C) utilize alpha- and beta-linked glucans, such as starch, barley glucan, laminarin, and chitin, while hyperthermophilic marine bacteria (T(opt)>or=80 degrees C) utilize the same glucans as well as hemicellulose, such as xylans and mannans. However, none of these organisms are able to efficiently utilize crystalline cellulose. Among the thermophiles, this ability is limited to a few terrestrial bacteria with upper temperature limits for growth near 75 degrees C. Deconstruction of crystalline cellulose by these extreme thermophiles is achieved by 'free' primary cellulases, which are distinct from those typically associated with large multi-enzyme complexes known as cellulosomes. These primary cellulases also differ from the endoglucanases (referred to here as 'secondary cellulases') reported from marine hyperthermophiles that show only weak activity toward cellulose. Many extremely thermophilic enzymes implicated in the deconstruction of lignocellulose can be identified in genome sequences, and many more promising biocatalysts probably remain annotated as 'hypothetical proteins'. Characterization of these enzymes will require intensive effort but is likely to generate new opportunities for the use of renewable resources as biofuels.

Genomic analyses and the origin of the eukaryotes

The availability of whole-genome data has created the extraordinary opportunity to reconstruct in fine details the 'tree of life'. The application of such comprehensive effort promises to unravel the enigmatic evolutionary relationships between prokaryotes and eukaryotes. Traditionally, biologists have represented the evolutionary relationships of all organisms by a bifurcating phylogenetic tree. But recent analyses of completely sequenced genomes using conditioned reconstruction (CR), a newly developed gene-content algorithm, suggest that a cycle graph or 'ring' rather than a 'tree' is a better representation of the evolutionary relationships between prokaryotes and eukaryotes. CR is the first phylogenetic-reconstruction method to provide precise evidence about the origin of the eukaryotes. This review summarizes how the CR analyses of complete genomes provide evidence for a fusion origin of the eukaryotes.

Structural and transcriptional analyses of a purine nucleotide-binding protein from Pyrococcus furiosus: a component of a novel, membrane-bound multiprotein complex unique to this hyperthermophilic archaeon

The open-reading frame PF0895 in the genome of the hyperthermophilic archaeon, Pyrococcus furiosus, encodes a 206-residue protein (M(R )23,152). The structure of the recombinant protein was solved by single isomorphous replacement with anomalous scattering (SIRAS) using a mercury derivative. It has been refined to 1.70 A with a crystallographic R and R(free )values of 19.7% and 22.3%, respectively. The PF0895 structure is similar to those of the ATP binding cassettes observed in the ABC transporter family. However, bioinformatics and molecular analyses indicate that PF0895 is not part of the expected five-gene operon that encodes a typical prokaryotic solute-binding ABC transporter. Rather, transcriptional profiling data show that PF0895 is part of a novel four-gene operon (PF0895-PF0896-PF0897-PF0897.1) where only PF0895 has homologs in other organisms. Interestingly, from genome analysis, P. furiosus itself contains a second version of this complex, encoded by PF1090-PF1093. From the structural studies we can only conclude that one of the subunits of this novel membrane complex, PF0895, and its homolog PF1090, likely bind a purine nucleotide. PF0895 is therefore predicted to be part of a membrane-bound multiprotein complex unrelated to ABC transporters that is so far unique to P. furiosus. It appears to play a role in the stress response, as its expression is down regulated when the organism is subjected to cold-shock, where cells are transferred from 95 degrees C, near the optimal growth temperature, to 72 degrees C, near the minimal growth temperature. The related PF1090-containing operon is unaffected by cold-shock and is independently regulated.

Insertion sequence diversity in archaea

Insertion sequences (ISs) can constitute an important component of prokaryotic (bacterial and archaeal) genomes. Over 1,500 individual ISs are included at present in the ISfinder database (www-is.biotoul.fr), and these represent only a small portion of those in the available prokaryotic genome sequences and those that are being discovered in ongoing sequencing projects. In spite of this diversity, the transposition mechanisms of only a few of these ubiquitous mobile genetic elements are known, and these are all restricted to those present in bacteria. This review presents an overview of ISs within the archaeal kingdom. We first provide a general historical summary of the known properties and behaviors of archaeal ISs. We then consider how transposition might be regulated in some cases by small antisense RNAs and by termination codon readthrough. This is followed by an extensive analysis of the IS content in the sequenced archaeal genomes present in the public databases as of June 2006, which provides an overview of their distribution among the major archaeal classes and species. We show that the diversity of archaeal ISs is very great and comparable to that of bacteria. We compare archaeal ISs to known bacterial ISs and find that most are clearly members of families first described for bacteria. Several cases of lateral gene transfer between bacteria and archaea are clearly documented, notably for methanogenic archaea. However, several archaeal ISs do not have bacterial equivalents but can be grouped into Archaea-specific groups or families. In addition to ISs, we identify and list nonautonomous IS-derived elements, such as miniature inverted-repeat transposable elements. Finally, we present a possible scenario for the evolutionary history of ISs in the Archaea.

The impact of extremophiles on structural genomics (and vice versa)

The advent of the complete genome sequences of various organisms in the mid-1990s raised the issue of how one could determine the function of hypothetical proteins. While insight might be obtained from a 3D structure, the chances of being able to predict such a structure is limited for the deduced amino acid sequence of any uncharacterized gene. A template for modeling is required, but there was only a low probability of finding a protein closely-related in sequence with an available structure. Thus, in the late 1990s, an international effort known as structural genomics (SG) was initiated, its primary goal to "fill sequence-structure space" by determining the 3D structures of representatives of all known protein families. This was to be achieved mainly by X-ray crystallography and it was estimated that at least 5,000 new structures would be required. While the proteins (genes) for SG have subsequently been derived from hundreds of different organisms, extremophiles and particularly thermophiles have been specifically targeted due to the increased stability and ease of handling of their proteins, relative to those from mesophiles. This review summarizes the significant impact that extremophiles and proteins derived from them have had on SG projects worldwide. To what extent SG has influenced the field of extremophile research is also discussed.

Functional characterization of a NapA Na+/H+antiporter fromThermus thermophilus

Na(+)/H(+) antiporters are ubiquitous membrane proteins and play an important role in cell homeostasis. We amplified a gene encoding a member of the monovalent cation:proton antiporter-2 (CPA2) family (TC 2.A.37) from the Thermus thermophilus genome and expressed it in Escherichia coli. The gene product was identified as a member of the NapA subfamily and was found to be an active Na(+)(Li(+))/H(+) antiporter as it conferred resistance to the Na(+) and Li(+) sensitive strain E. coli EP432 (DeltanhaA, DeltanhaB) upon exposure to high concentration of these salts in the growth medium. Fluorescence measurements using the pH sensitive dye 9-amino-6-chloro-2-methoxyacridine in everted membrane vesicles of complemented E. coli EP432 showed high Li(+)/H(+) exchange activity at pH 6, but marginal Na(+)/H(+) antiport activity. Towards more alkaline conditions, Na(+)/H(+) exchange activity increased to a relative maximum at pH 8, where by contrast the Li(+)/H(+) exchange activity reached its relative minimum. Substitution of conserved residues D156 and D157 (located in the putative transmembrane helix 6) with Ala resulted in the complete loss of Na(+)/H(+) activity. Mutation of K305 (putative transmembrane helix 10) to Ala resulted in a compromised phenotype characterized by an increase in apparent K(m) for Na(+) (36 vs. 7.6 mM for the wildtype) and Li(+) (17 vs. 0.22 mM), In summary, the Na(+)/H(+) antiport activity profile of the NapA type transporter of T. thermophilus resembles that of NhaA from E. coli, whereas in contrast to NhaA the T. thermophilus NapA antiporter is characterized by high Li(+)/H(+) antiport activity at acidic pH.

A Compression-Based Approach for Coding Sequences Identification. I. Application to Prokaryotic Genomes

Most of the gene prediction algorithms for prokaryotes are based on Hidden Markov Models or similar machine-learning approaches, which imply the optimization of a high number of parameters. The present paper presents a novel method for the classification of coding and non-coding regions in prokaryotic genomes, based on a suitably defined compression index of a DNA sequence. The main features of this new method are the non-parametric logic and the costruction of a dictionary of words extracted from the sequences. These dictionaries can be very useful to perform further analyses on the genomic sequences themselves. The proposed approach has been applied on some prokaryotic complete genomes, obtaining optimal scores of correctly recognized coding and non-coding regions. Several false-positive and false-negative cases have been investigated in detail, which have revealed that this approach can fail in the presence of highly structured coding regions (e.g., genes coding for modular proteins) or quasi-random non-coding regions (e.g., regions hosting non-functional fragments of copies of functional genes; regions hosting promoters or other protein-binding sequences). We perform an overall comparison with other gene-finder software, since at this step we are not interested in building another gene-finder system, but only in exploring the possibility of the suggested approach.

Chromosome evolution in the Thermotogales: large-scale inversions and strain diversification of CRISPR sequences

In the present study, the chromosomes of two members of the Thermotogales were compared. A whole-genome alignment of Thermotoga maritima MSB8 and Thermotoga neapolitana NS-E has revealed numerous large-scale DNA rearrangements, most of which are associated with CRISPR DNA repeats and/or tRNA genes. These DNA rearrangements do not include the putative origin of DNA replication but move within the same replichore, i.e., the same replicating half of the chromosome (delimited by the replication origin and terminus). Based on cumulative GC skew analysis, both the T. maritima and T. neapolitana lineages contain one or two major inverted DNA segments. Also, based on PCR amplification and sequence analysis of the DNA joints that are associated with the major rearrangements, the overall chromosome architecture was found to be conserved at most DNA joints for other strains of T. neapolitana. Taken together, the results from this analysis suggest that the observed chromosomal rearrangements in the Thermotogales likely occurred by successive inversions after their divergence from a common ancestor and before strain diversification. Finally, sequence analysis shows that size polymorphisms in the DNA joints associated with CRISPRs can be explained by expansion and possibly contraction of the DNA repeat and spacer unit, providing a tool for discerning the relatedness of strains from different geographic locations.

Metabolic and evolutionary relationships among Pyrococcus Species: genetic exchange within a hydrothermal vent environment

Pyrococcus furiosus and Pyrococcus woesei grow optimally at temperatures near 100 degrees C and were isolated from the same shallow marine volcanic vent system. Hybridization of genomic DNA from P. woesei to a DNA microarray containing all 2,065 open reading frames (ORFs) annotated in the P. furiosus genome, in combination with PCR analysis, indicated that homologs of 105 ORFs present in P. furiosus are absent from the uncharacterized genome of P. woesei. Pulsed-field electrophoresis indicated that the sizes of the two genomes are comparable, and the results were consistent with the hypothesis that P. woesei lacks the 105 ORFs found in P. furiosus. The missing ORFs are present in P. furiosus mainly in clusters. These clusters include one cluster (Mal I, PF1737 to PF1751) involved in maltose metabolism and another cluster (PF0691 to PF0695) whose products are thought to remove toxic reactive nitrogen species. Accordingly, it was found that P. woesei, in contrast to P. furiosus, is unable to utilize maltose as a carbon source for growth, and the growth of P. woesei on starch was inhibited by addition of a nitric oxide generator. In P. furiosus the ORF clusters not present in P. woesei are bracketed by or are in the vicinity of insertion sequences or long clusters of tandem repeats (LCTRs). While the role of LCTRs in lateral gene transfer is not known, the Mal I cluster in P. furiosus is a composite transposon that undergoes replicative transposition. The same locus in P. woesei lacks any evidence of insertion activity, indicating that P. woesei is a sister or even the parent of P. furiosus. P. woesei may have acquired by lateral gene transfer more than 100 ORFs from other organisms living in the same thermophilic environment to produce the type strain of P. furiosus.

Cloning, purification, crystallization and preliminary crystallographic analysis of a penicillin-binding protein homologue from Pyrococcus abyssi

The genome of the hyperthermophilic archaeon Pyrococcus abyssi contains a gene (pab0087) encoding a penicillin-binding protein (PBP) homologue. This sequence consists of 447 residues and shows significant sequence similarity to low-molecular-weight PBPs and class C beta-lactamases. The Pab0087 protein was overexpressed, purified and crystallized. Diffraction data from two different crystal forms were collected to 2.7 and 2.0 A resolution. Both crystals belong to space group C2, with unit-cell parameters a = 160.59, b = 135.74, c = 113.02 A, beta = 117.36 degrees and a = 166.97, b = 131.25, c = 189.39 A, beta = 113.81 degrees , respectively. The asymmetric unit contains four and eight molecules, respectively, with fourfold non-crystallographic symmetry.

Cloning, purification and crystallization of a Walker-type Pyrococcus abyssi ATPase family member

Several ATPase proteins play essential roles in the initiation of chromosomal DNA replication in archaea. Walker-type ATPases are defined by their conserved Walker A and B motifs, which are associated with nucleotide binding and ATP hydrolysis. A family of 28 ATPase proteins with non-canonical Walker A sequences has been identified by a bioinformatics study of comparative genomics in Pyrococcus genomes. A high-throughput structural study on P. abyssi has been started in order to establish the structure of these proteins. 16 genes have been cloned and characterized. Six out of the seven soluble constructs were purified in Escherichia coli and one of them, PABY2304, has been crystallized. X-ray diffraction data were collected from selenomethionine-derivative crystals using synchrotron radiation. The crystals belong to the orthorhombic space group C2, with unit-cell parameters a = 79.41, b = 48.63, c = 108.77 A, and diffract to beyond 2.6 A resolution.

Evidence for genetic drift in the diversification of a geographically isolated population of the hyperthermophilic archaeon Pyrococcus

Genetic drift is a mechanism of population divergence that is important in the evolution of plants and animals but is thought to be rare in free-living microorganisms because of their typically large population sizes and unrestricted means of dispersal. We used both phylogenetic and insertion sequence (IS) element analyses in hyperthermophilic archaea of the genus Pyrococcus to test the hypothesis that genetic drift played an important role in the diversification of these microorganisms. Multilocus sequence typing of a collection of 36 isolates of Pyrococcus, from different hydrothermal systems in the Pacific Ocean and the Mediterranean Sea, revealed that Pyrococcus populations from different geographic locations are genetically differentiated. Analysis of IS elements in these isolates exposed their presence in all individuals of only one geographically isolated lineage, that of Vulcano Island in the Mediterranean Sea. Detailed sequence analysis of six selected IS elements in the Vulcano population showed that these elements cause deleterious genomic alterations, including inactivation of gene function. The high frequency of IS elements in the sampled population together with their observed harmful effects in the genome of Pyrococcus provide molecular evidence that the Vulcano Island population of Pyrococcus is geographically isolated and that those genetic mobile elements have been brought up to high frequency by genetic drift. Thus, genetic drift resulting from physical isolation should be considered as a factor influencing differentiation in prokaryotes.

A proposal to rename the hyperthermophile Pyrococcus woesei as Pyrococcus furiosus subsp. woesei

Pyrococcus species are hyperthermophilic members of the order Thermococcales, with optimal growth temperatures approaching 100 degrees C. All species grow heterotrophically and produce H2 or, in the presence of elemental sulfur (S(o)), H2S. Pyrococcus woesei and P. furiosus were isolated from marine sediments at the same Vulcano Island beach site and share many morphological and physiological characteristics. We report here that the rDNA operons of these strains have identical sequences, including their intergenic spacer regions and part of the 23S rRNA. Both species grow rapidly and produce H2 in the presence of 0.1% maltose and 10-100 microM sodium tungstate in S(o)-free medium. However, P. woesei shows more extensive autolysis than P. furiosus in the stationary phase. Pyrococcus furiosus and P. woesei share three closely related families of insertion sequences (ISs). A Southern blot performed with IS probes showed extensive colinearity between the genomes of P. woesei and P. furiosus. Cloning and sequencing of ISs that were in different contexts in P. woesei and P. furiosus revealed that the napA gene in P. woesei is disrupted by a type III IS element, whereas in P. furiosus, this gene is intact. A type I IS element, closely linked to the napA gene, was observed in the same context in both P. furiosus and P. woesei genomes. Our results suggest that the IS elements are implicated in genomic rearrangements and reshuffling in these closely related strains. We propose to rename P. woesei a subspecies of P. furiosus based on their identical rDNA operon sequences, many common IS elements that are shared genomic markers, and the observation that all P. woesei nucleotide sequences deposited in GenBank to date are > 99% identical to P. furiosus sequences.

Identification of replication origins in archaeal genomes based on the Z-curve method

The Z-curve is a three-dimensional curve that constitutes a unique representation of a DNA sequence, i.e., both the Z-curve and the given DNA sequence can be uniquely reconstructed from the other. We employed Z-curve analysis to identify one replication origin in the Methanocaldococcus jannaschii genome, two replication origins in the Halobacterium species NRC-1 genome and one replication origin in the Methanosarcina mazei genome. One of the predicted replication origins of Halobacterium species NRC-1 is the same as a replication origin later identified by in vivo experiments. The Z-curve analysis of the Sulfolobus solfataricus P2 genome suggested the existence of three replication origins, which is also consistent with later experimental results. This review aims to summarize applications of the Z-curve in identifying replication origins of archaeal genomes, and to provide clues about the locations of as yet unidentified replication origins of the Aeropyrum pernix K1, Methanococcus maripaludis S2, Picrophilus torridus DSM 9790 and Pyrobaculum aerophilum str. IM2 genomes.

Decoding the genomic tree of life

Genomes hold within them the record of the evolution of life on Earth. But genome fusions and horizontal gene transfer (HGT) seem to have obscured sufficiently the gene sequence record such that it is difficult to reconstruct the phylogenetic tree of life. HGT among prokaryotes is not random, however. Some genes (informational genes) are more difficult to transfer than others (operational genes). Furthermore, environmental, metabolic, and genetic differences among organisms restrict HGT, so that prokaryotes preferentially share genes with other prokaryotes having properties in common, including genome size, genome G+C composition, carbon utilization, oxygen utilization/sensitivity, and temperature optima, further complicating attempts to reconstruct the tree of life. A new method of phylogenetic reconstruction based on gene presence and absence, called conditioned reconstruction, has improved our prospects for reconstructing prokaryotic evolution. It is also able to detect past genome fusions, such as the fusion that appears to have created the first eukaryote. This genome fusion between a deep branching eubacterium, possibly an ancestor of the cyanobacterium and a proteobacterium, with an archaeal eocyte (crenarchaea), appears to be the result of an early symbiosis. Given new tools and new genes from relevant organisms, it should soon be possible to test current and future fusion theories for the origin of eukaryotes and to discover the general outlines of the prokaryotic tree of life.

Unique biosynthesis of dehydroquinic acid?

A search of the genomic sequences of the thermophilic microorganisms Aquifex aeolicus, Archaeoglobus fulgidus, Methanobacterium thermoautotrophicum, and Methanococcus jannaschii for the first seven enzymes (aroG, B, D, E, K, A, and C ) involved in the shikimic acid biosynthetic pathway reveal two key enzymes are missing. The first enzyme in the pathway, 3-deoxy-d-arabino-heptulosonic acid 7-phosphate synthase (aroG) and the second enzyme in the pathway, 5-dehydroquinic acid synthase (aroB) are "missing." The remaining five genes for the shikimate pathway in these organism are present and are similar to the corresponding Escherichia coli genes. The genomic sequences of the thermophiles Pyrococcus abyssi and Thermotoga maritima contain the aroG and aroB genes. Several fungi such as Aspergillus fumigatus, Aspergillus nidulans, Saccharomyces cerevisiae, Schizosaccharomyces pombe, Pneumocystis carinii f. sp. carinii, and Neurospora crassa contain the gene aroM, a pentafunctional enzyme whose overall activity is equivalent to the combined catalytic activities of proteins expressed by aroB, D, E, K, and A genes. Two of these fungi also lack an aroG gene. A discussion of potential reasons for these missing enzymes is presented.

Complete genome sequence of the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 and comparison with Pyrococcus genomes

The genus Thermococcus, comprised of sulfur-reducing hyperthermophilic archaea, belongs to the order Thermococcales in Euryarchaeota along with the closely related genus Pyrococcus. The members of Thermococcus are ubiquitously present in natural high-temperature environments, and are therefore considered to play a major role in the ecology and metabolic activity of microbial consortia within hot-water ecosystems. To obtain insight into this important genus, we have determined and annotated the complete 2,088,737-base genome of Thermococcus kodakaraensis strain KOD1, followed by a comparison with the three complete genomes of Pyrococcus spp. A total of 2306 coding DNA sequences (CDSs) have been identified, among which half (1165 CDSs) are annotatable, whereas the functions of 41% (936 CDSs) cannot be predicted from the primary structures. The genome contains seven genes for probable transposases and four virus-related regions. Several proteins within these genetic elements show high similarities to those in Pyrococcus spp., implying the natural occurrence of horizontal gene transfer of such mobile elements among the order Thermococcales. Comparative genomics clarified that 1204 proteins, including those for information processing and basic metabolisms, are shared among T. kodakaraensis and the three Pyrococcus spp. On the other hand, among the set of 689 proteins unique to T. kodakaraensis, there are several intriguing proteins that might be responsible for the specific trait of the genus Thermococcus, such as proteins involved in additional pyruvate oxidation, nucleotide metabolisms, unique or additional metal ion transporters, improved stress response system, and a distinct restriction system.

Comparative genomic hybridizations of Entamoeba strains reveal unique genetic fingerprints that correlate with virulence

Variable phenotypes have been identified for Entamoeba species. Entamoeba histolytica is invasive and causes colitis and liver abscesses but only in approximately 10% of infected individuals; 90% remain asymptomatically colonized. Entamoeba dispar, a closely related species, is avirulent. To determine the extent of genetic diversity among Entamoeba isolates and potential genotype-phenotype correlations, we have developed an E. histolytica genomic DNA microarray and used it to genotype strains of E. histolytica and E. dispar. On the basis of the identification of divergent genetic loci, all strains had unique genetic fingerprints. Comparison of divergent genetic regions allowed us to distinguish between E. histolytica and E. dispar, identify novel genetic regions usable for strain and species typing, and identify a number of genes restricted to virulent strains. Among the four E. histolytica strains, a strain with attenuated virulence was the most divergent and phylogenetically distinct strain, raising the intriguing possibility that genetic subtypes of E. histolytica may be partially responsible for the observed variability in clinical outcomes. This microarray-based genotyping assay can readily be applied to the study of E. histolytica clinical isolates to determine genetic diversity and potential genotypic-phenotypic associations.

Molecular diversity of new Thermococcales isolates from a single area of hydrothermal deep-sea vents as revealed by randomly amplified polymorphic DNA fingerprinting and 16S rRNA gene sequence analysis

Members of the Thermococcales are anaerobic Archaea belonging to the kingdom Euryarchaea that are studied in many laboratories as model organisms for hyperthermophiles. We describe here a molecular analysis of 86 new Thermococcales isolates collected from six different chimneys of a single hydrothermal field located in the 13 degrees N 104 degrees W segment of the East Pacific ridge at a depth of 2,330 m. These isolates were sorted by randomly amplified polymorphic DNA (RAPD) fingerprinting into nine groups, and nine unique RAPD profiles were obtained. One RAPD group corresponds to new isolates of Thermococcus hydrothermalis, whereas all other groups and isolates with unique profiles are different from the 22 reference strains included in this study. Analysis of 16S rRNA gene sequences of representatives of each RAPD group and unique profiles showed that one group corresponds to Pyrococcus strains, whereas all the other isolates are Thermococcus strains. We estimated that our collection may contain at least 11 new species. These putative species, isolated from a single area of hydrothermal deep-sea vents, are dispersed in the 16S rRNA tree among the reference strains previously isolated from diverse hot environments (terrestrial, shallow water, hydrothermal vents) located around the world, suggesting that there is a high degree of dispersal of Thermococcales: About one-half of our isolates contain extrachromosomal elements that could be used to search for novel replication proteins and to develop genetic tools for hyperthermophiles.

Analysis of genes that encode DtxR-like transcriptional regulators in pathogenic and saprophytic corynebacterial species

Metal-dependent transcriptional regulators of the diphtheria toxin repressor (DtxR) family have been identified in a wide variety of bacterial genera, where they control gene expression in response to one of two metal ions, Fe(2+) or Mn(2+). DtxR of Corynebacterium diphtheriae is the best characterized of these important metal-dependent regulators. The genus Corynebacterium includes many phenotypically diverse species, and the prevalence of DtxR-like regulators within the genus is unknown. We assayed chromosomal DNA from 42 different corynebacterial isolates, representing 33 different species, for the presence of a highly conserved region of the dtxR gene that encodes the DNA-binding helix-turn-helix motif and metal-binding site 1 within domains 1 and 2 of DtxR. The chromosome of all of the isolates contained this conserved region of dtxR, and DNA sequencing revealed a high level of nucleotide sequence conservation within this region in all of the corynebacterial species (ranging from 62 to 100% identity and averaging 70% identity with the dtxR prototype). The level of identity was even greater for the predicted protein sequences encoded by the dtxR-like genes, ranging from 81 to 100% identity and averaging 91% identity with DtxR. Using a DtxR-specific antiserum we confirmed the presence of a DtxR-like protein in extracts of most of the corynebacterial isolates and determined the precise amount of DtxR per cell in C. diphtheriae. The high level of identity at both DNA and protein levels suggests that all of the isolates tested encode a functional DtxR-like Fe(2+)-activated regulatory protein that can bind homologs of the DtxR operator and regulate gene expression in response to iron.

Divergence and redundancy of 16S rRNA sequences in genomes with multiple rrn operons

The level of sequence heterogeneity among rrn operons within genomes determines the accuracy of diversity estimation by 16S rRNA-based methods. Furthermore, the occurrence of widespread horizontal gene transfer (HGT) between distantly related rrn operons casts doubt on reconstructions of phylogenetic relationships. For this study, patterns of distribution of rrn copy numbers, interoperonic divergence, and redundancy of 16S rRNA sequences were evaluated. Bacterial genomes display up to 15 operons and operon numbers up to 7 are commonly found, but approximately 40% of the organisms analyzed have either one or two operons. Among the Archaea, a single operon appears to dominate and the highest number of operons is five. About 40% of sequences among 380 operons in 76 bacterial genomes with multiple operons were identical to at least one other 16S rRNA sequence in the same genome, and in 38% of the genomes all 16S rRNAs were invariant. For Archaea, the number of identical operons was only 25%, but only five genomes with 21 operons are currently available. These considerations suggest an upper bound of roughly threefold overestimation of bacterial diversity resulting from cloning and sequencing of 16S rRNA genes from the environment; however, the inclusion of genomes with a single rrn operon may lower this correction factor to approximately 2.5. Divergence among operons appears to be small overall for both Bacteria and Archaea, with the vast majority of 16S rRNA sequences showing <1% nucleotide differences. Only five genomes with operons with a higher level of nucleotide divergence were detected, and Thermoanaerobacter tengcongensis exhibited the highest level of divergence (11.6%) noted to date. Overall, four of the five extreme cases of operon differences occurred among thermophilic bacteria, suggesting a much higher incidence of HGT in these bacteria than in other groups.

A comprehensive evolutionary classification of proteins encoded in complete eukaryotic genomes

We examined functional and evolutionary patterns in the recently constructed set of 5,873 clusters of predicted orthologs from seven eukaryotic genomes. The analysis reveals a conserved core of largely essential eukaryotic genes as well as major diversification and innovation associated with evolution of eukaryotic genomes.

Background

Sequencing the genomes of multiple, taxonomically diverse eukaryotes enables in-depth comparative-genomic analysis which is expected to help in reconstructing ancestral eukaryotic genomes and major events in eukaryotic evolution and in making functional predictions for currently uncharacterized conserved genes.

Results

We examined functional and evolutionary patterns in the recently constructed set of 5,873 clusters of predicted orthologs (eukaryotic orthologous groups or KOGs) from seven eukaryotic genomes: Caenorhabditis elegans, Drosophila melanogaster, Homo sapiens, Arabidopsis thaliana, Saccharomyces cerevisiae, Schizosaccharomyces pombe and Encephalitozoon cuniculi. Conservation of KOGs through the phyletic range of eukaryotes strongly correlates with their functions and with the effect of gene knockout on the organism's viability. The approximately 40% of KOGs that are represented in six or seven species are enriched in proteins responsible for housekeeping functions, particularly translation and RNA processing. These conserved KOGs are often essential for survival and might approximate the minimal set of essential eukaryotic genes. The 131 single-member, pan-eukaryotic KOGs we identified were examined in detail. For around 20 that remained uncharacterized, functions were predicted by in-depth sequence analysis and examination of genomic context. Nearly all these proteins are subunits of known or predicted multiprotein complexes, in agreement with the balance hypothesis of evolution of gene copy number. Other KOGs show a variety of phyletic patterns, which points to major contributions of lineage-specific gene loss and the 'invention' of genes new to eukaryotic evolution. Examination of the sets of KOGs lost in individual lineages reveals co-elimination of functionally connected genes. Parsimonious scenarios of eukaryotic genome evolution and gene sets for ancestral eukaryotic forms were reconstructed. The gene set of the last common ancestor of the crown group consists of 3,413 KOGs and largely includes proteins involved in genome replication and expression, and central metabolism. Only 44% of the KOGs, mostly from the reconstructed gene set of the last common ancestor of the crown group, have detectable homologs in prokaryotes; the remainder apparently evolved via duplication with divergence and invention of new genes.

Conclusions

The KOG analysis reveals a conserved core of largely essential eukaryotic genes as well as major diversification and innovation associated with evolution of eukaryotic genomes. The results provide quantitative support for major trends of eukaryotic evolution noticed previously at the qualitative level and a basis for detailed reconstruction of evolution of eukaryotic genomes and biology of ancestral forms.

The COG database: an updated version includes eukaryotes

BACKGROUND

The availability of multiple, essentially complete genome sequences of prokaryotes and eukaryotes spurred both the demand and the opportunity for the construction of an evolutionary classification of genes from these genomes. Such a classification system based on orthologous relationships between genes appears to be a natural framework for comparative genomics and should facilitate both functional annotation of genomes and large-scale evolutionary studies.

RESULTS

We describe here a major update of the previously developed system for delineation of Clusters of Orthologous Groups of proteins (COGs) from the sequenced genomes of prokaryotes and unicellular eukaryotes and the construction of clusters of predicted orthologs for 7 eukaryotic genomes, which we named KOGs after eukaryotic orthologous groups. The COG collection currently consists of 138,458 proteins, which form 4873 COGs and comprise 75% of the 185,505 (predicted) proteins encoded in 66 genomes of unicellular organisms. The eukaryotic orthologous groups (KOGs) include proteins from 7 eukaryotic genomes: three animals (the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster and Homo sapiens), one plant, Arabidopsis thaliana, two fungi (Saccharomyces cerevisiae and Schizosaccharomyces pombe), and the intracellular microsporidian parasite Encephalitozoon cuniculi. The current KOG set consists of 4852 clusters of orthologs, which include 59,838 proteins, or approximately 54% of the analyzed eukaryotic 110,655 gene products. Compared to the coverage of the prokaryotic genomes with COGs, a considerably smaller fraction of eukaryotic genes could be included into the KOGs; addition of new eukaryotic genomes is expected to result in substantial increase in the coverage of eukaryotic genomes with KOGs. Examination of the phyletic patterns of KOGs reveals a conserved core represented in all analyzed species and consisting of approximately 20% of the KOG set. This conserved portion of the KOG set is much greater than the ubiquitous portion of the COG set (approximately 1% of the COGs). In part, this difference is probably due to the small number of included eukaryotic genomes, but it could also reflect the relative compactness of eukaryotes as a clade and the greater evolutionary stability of eukaryotic genomes.

CONCLUSION

The updated collection of orthologous protein sets for prokaryotes and eukaryotes is expected to be a useful platform for functional annotation of newly sequenced genomes, including those of complex eukaryotes, and genome-wide evolutionary studies.

PipeAlign: A new toolkit for protein family analysis

PipeAlign is a protein family analysis tool integrating a five step process ranging from the search for sequence homologues in protein and 3D structure databases to the definition of the hierarchical relationships within and between subfamilies. The complete, automatic pipeline takes a single sequence or a set of sequences as input and constructs a high-quality, validated MACS (multiple alignment of complete sequences) in which sequences are clustered into potential functional subgroups. For the more experienced user, the PipeAlign server also provides numerous options to run only a part of the analysis, with the possibility to modify the default parameters of each software module. For example, the user can choose to enter an existing multiple sequence alignment for refinement, validation and subsequent clustering of the sequences. The aim is to provide an interactive workbench for the validation, integration and presentation of a protein family, not only at the sequence level, but also at the structural and functional levels. PipeAlign is available at http://igbmc.u-strasbg.fr/PipeAlign/.

An integrated analysis of the genome of the hyperthermophilic archaeon Pyrococcus abyssi

The hyperthermophilic euryarchaeon Pyrococcus abyssi and the related species Pyrococcus furiosus and Pyrococcus horikoshii, whose genomes have been completely sequenced, are presently used as model organisms in different laboratories to study archaeal DNA replication and gene expression and to develop genetic tools for hyperthermophiles. We have performed an extensive re-annotation of the genome of P. abyssi to obtain an integrated view of its phylogeny, molecular biology and physiology. Many new functions are predicted for both informational and operational proteins. Moreover, several candidate genes have been identified that might encode missing links in key metabolic pathways, some of which have unique biochemical features. The great majority of Pyrococcus proteins are typical archaeal proteins and their phylogenetic pattern agrees with its position near the root of the archaeal tree. However, proteins probably from bacterial origin, including some from mesophilic bacteria, are also present in the P. abyssi genome.

Prediction of novel archaeal enzymes from sequence-derived features

The completely sequenced archaeal genomes potentially encode, among their many functionally uncharacterized genes, novel enzymes of biotechnological interest. We have developed a prediction method for detection and classification of enzymes from sequence alone (available at http://www.cbs.dtu.dk/services/ArchaeaFun/). The method does not make use of sequence similarity; rather, it relies on predicted protein features like cotranslational and posttranslational modifications, secondary structure, and simple physical/chemical properties.

Genomic BLAST: custom-defined virtual databases for complete and unfinished genomes

BLAST (Basic Local Alignment Search Tool) searches against DNA and protein sequence databases have become an indispensable tool for biomedical research. The proliferation of the genome sequencing projects is steadily increasing the fraction of genome-derived sequences in the public databases and their importance as a public resource. We report here the availability of Genomic BLAST, a novel graphical tool for simplifying BLAST searches against complete and unfinished genome sequences. This tool allows the user to compare the query sequence against a virtual database of DNA and/or protein sequences from a selected group of organisms with finished or unfinished genomes. The organisms for such a database can be selected using either a graphic taxonomy-based tree or an alphabetical list of organism-specific sequences. The first option is designed to help explore the evolutionary relationships among organisms within a certain taxonomy group when performing BLAST searches. The use of an alphabetical list allows the user to perform a more elaborate set of selections, assembling any given number of organism-specific databases from unfinished or complete genomes. This tool, available at the NCBI web site http://www.ncbi.nlm.nih.gov/cgi-bin/Entrez/genom_table_cgi, currently provides access to over 170 bacterial and archaeal genomes and over 40 eukaryotic genomes.

Construction of a shuttle vector for, and spheroplast transformation of, the hyperthermophilic archaeon Pyrococcus abyssi

Our understanding of the genetics of species of the best-studied hyperthermophilic archaea, Pyrococcus spp., is presently limited by the lack of suitable genetic tools, such as a stable cloning vector and the ability to select individual transformants on plates. Here we describe the development of a reliable host-vector system for the hyperthermophilic archaeon Pyrococcus abyssi. Shuttle vectors were constructed based on the endogenous plasmid pGT5 from P. abyssi strain GE5 and the bacterial vector pLitmus38. As no antibiotic resistance marker is currently available for Pyrococcus spp., we generated a selectable auxotrophic marker. Uracil auxotrophs resistant to 5-fluoorotic acid were isolated from P. abyssi strain GE9 (devoid of pGT5). Genetic analysis of these mutants revealed mutations in the pyrE and/or pyrF genes, encoding key enzymes of the pyrimidine biosynthetic pathway. Two pyrE mutants exhibiting low reversion rates were retained for complementation experiments. For that purpose, the pyrE gene, encoding orotate phosphoribosyltransferase (OPRTase) of the thermoacidophilic crenarchaeote Sulfolobus acidocaldarius, was introduced into the pGT5-based vector, giving rise to pYS2. With a polyethylene glycol-spheroplast method, we could reproducibly transform P. abyssi GE9 pyrE mutants to prototrophy, though with low frequency (10(2) to 10(3) transformants per micro g of pYS2 plasmid DNA). Transformants did grow as well as the wild type on minimal medium without uracil and showed comparable OPRTase activity. Vector pYS2 proved to be very stable and was maintained at high copy number under selective conditions in both Escherichia coli and P. abyssi.

A comparative genomic method for computational identification of prokaryotic translation initiation sites

The ever growing number of completely sequenced prokaryotic genomes facilitates cross-species comparisons by genomic annotation algorithms. This paper introduces a new probabilistic framework for comparative genomic analysis and demonstrates its utility in the context of improving the accuracy of prokaryotic gene start site detection. Our frame work employs a product hidden Markov model (PROD-HMM) with state architecture to model the species-specific trinucleotide frequency patterns in sequences immediately upstream and downstream of a translation start site and to detect the contrasting non-synonymous (amino acid changing) and synonymous (silent) substitution rates that differentiate prokaryotic coding from intergenic regions. Depending on the intricacy of the features modeled by the hidden state architecture, intergenic, regulatory, promoter and coding regions can be delimited by this method. The new system is evaluated using a preliminary set of orthologous Pyrococcus gene pairs, for which it demonstrates an improved accuracy of detection. Its robustness is confirmed by analysis with cross-validation of an experimentally verified set of Escherichia coli K-12 and Salmonella thyphimurium LT2 orthologs. The novel architecture has a number of attractive features that distinguish it from previous comparative models such as pair-HMMs.

Horizontal gene transfer in microbial genome evolution

Horizontal gene transfer is the collective name for processes that permit the exchange of DNA among organisms of different species. Only recently has it been recognized as a significant contribution to inter-organismal gene exchange. Traditionally, it was thought that microorganisms evolved clonally, passing genes from mother to daughter cells with little or no exchange of DNA among diverse species. Studies of microbial genomes, however, have shown that genomes contain genes that are closely related to a number of different prokaryotes, sometimes to phylogenetically very distantly related ones. (Doolittle et al., 1990, J. Mol. Evol. 31, 383-388; Karlin et al., 1997, J. Bacteriol. 179, 3899-3913; Karlin et al., 1998, Annu. Rev. Genet. 32, 185-225; Lawrence and Ochman, 1998, Proc. Natl. Acad. Sci. USA 95, 9413-9417; Rivera et al., 1998, Proc. Natl. Acad. Sci. USA 95, 6239-6244; Campbell, 2000, Theor. Popul. Biol. 57 71-77; Doolittle, 2000, Sci. Am. 282, 90-95; Ochman and Jones, 2000, Embo. J. 19, 6637-6643; Boucher et al. 2001, Curr. Opin., Microbiol. 4, 285-289; Wang et al., 2001, Mol. Biol. Evol. 18, 792-800). Whereas prokaryotic and eukaryotic evolution was once reconstructed from a single 16S ribosomal RNA (rRNA) gene, the analysis of complete genomes is beginning to yield a different picture of microbial evolution, one that is wrought with the lateral movement of genes across vast phylogenetic distances. (Lane et al., 1988, Methods Enzymol. 167, 138-144; Lake and Rivera, 1996, Proc. Natl. Acad. Sci. USA 91, 2880-2881; Lake et al., 1999, Science 283, 2027-2028).