Towards understanding the first genome sequence of a crenarchaeon by genome annotation using clusters of orthologous groups of proteins (COGs).
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BACKGROUND: Standard archival sequence databases have not been designed as tools for genome annotation and are far from being optimal for this purpose. We used the database of Clusters of Orthologous Groups of proteins (COGs) to reannotate the genomes of two archaea, Aeropyrum pernix, the first member of the Crenarchaea to be sequenced, and Pyrococcus abyssi. RESULTS: A. pernix and P. abyssi proteins were assigned to COGs using the COGNITOR program; the results were verified on a case-by-case basis and augmented by additional database searches using the PSI-BLAST and TBLASTN programs. Functions were predicted for over 300 proteins from A. pernix, which could not be assigned a function using conventional methods with a conservative sequence similarity threshold, an approximately 50% increase compared to the original annotation. A. pernix shares most of the conserved core of proteins that were previously identified in the Euryarchaeota. Cluster analysis or distance matrix tree construction based on the co-occurrence of genomes in COGs showed that A. pernix forms a distinct group within the archaea, although grouping with the two species of Pyrococci, indicative of similar repertoires of conserved genes, was observed. No indication of a specific relationship between Crenarchaeota and eukaryotes was obtained in these analyses. Several proteins that are conserved in Euryarchaeota and most bacteria are unexpectedly missing in A. pernix, including the entire set of de novo purine biosynthesis enzymes, the GTPase FtsZ (a key component of the bacterial and euryarchaeal cell-division machinery), and the tRNA-specific pseudouridine synthase, previously considered universal. A. pernix is represented in 48 COGs that do not contain any euryarchaeal members. Many of these proteins are TCA cycle and electron transport chain enzymes, reflecting the aerobic lifestyle of A. pernix. CONCLUSIONS: Special-purpose databases organized on the basis of phylogenetic analysis and carefully curated with respect to known and predicted protein functions provide for a significant improvement in genome annotation. A differential genome display approach helps in a systematic investigation of common and distinct features of gene repertoires and in some cases reveals unexpected connections that may be indicative of functional similarities between phylogenetically distant organisms and of lateral gene exchange. (+info)
Interkingdom gene fusions.
(26/588)
BACKGROUND: Genome comparisons have revealed major lateral gene transfer between the three primary kingdoms of life - Bacteria, Archaea, and Eukarya. Another important evolutionary phenomenon involves the evolutionary mobility of protein domains that form versatile multidomain architectures. We were interested in investigating the possibility of a combination of these phenomena, with an invading gene merging with a pre-existing gene in the recipient genome. RESULTS: Complete genomes of fifteen bacteria, four archaea and one eukaryote were searched for interkingdom gene fusions (IKFs); that is, genes coding for proteins that apparently consist of domains originating from different primary kingdoms. Phylogenetic analysis supported 37 cases of IKF, each of which includes a 'native' domain and a horizontally acquired 'alien' domain. IKFs could have evolved via lateral transfer of a gene coding for the alien domain (or a larger protein containing this domain) followed by recombination with a native gene. For several IKFs, this scenario is supported by the presence of a gene coding for a second, stand-alone version of the alien domain in the recipient genome. Among the genomes investigated, the greatest number of IKFs has been detected in Mycobacterium tuberculosis, where they are almost always accompanied by a stand-alone alien domain. For most of the IKF cases detected in other genomes, the stand-alone counterpart is missing. CONCLUSIONS: The results of comparative genome analysis show that IKF formation is a real, but relatively rare, evolutionary phenomenon. We hypothesize that IKFs are formed primarily via the proposed two-stage mechanism, but other than in the Actinomycetes, in which IKF generation seems to be an active, ongoing process, most of the stand-alone intermediates have been eliminated, perhaps because of functional redundancy. (+info)
Extreme genomes.
(27/588)
The complete genome sequence of Thermoplasma acidophilum, an acid- and heat-loving archaeon, has recently been reported. Comparative genomic analysis of this 'extremophile' is providing new insights into the metabolic machinery, ecology and evolution of thermophilic archaea. (+info)
Genome alignment, evolution of prokaryotic genome organization, and prediction of gene function using genomic context.
(28/588)
Gene order in prokaryotes is conserved to a much lesser extent than protein sequences. Only several operons, primarily those that code for physically interacting proteins, are conserved in all or most of the bacterial and archaeal genomes. Nevertheless, even the limited conservation of operon organization that is observed can provide valuable evolutionary and functional clues through multiple genome comparisons. A program for constructing gapped local alignments of conserved gene strings in two genomes was developed. The statistical significance of the local alignments was assessed using Monte Carlo simulations. Sets of local alignments were generated for all pairs of completely sequenced bacterial and archaeal genomes, and for each genome a template-anchored multiple alignment was constructed. In most pairwise genome comparisons, <10% of the genes in each genome belonged to conserved gene strings. When closely related pairs of species (i.e., two mycoplasmas) are excluded, the total coverage of genomes by conserved gene strings ranged from <5% for the cyanobacterium Synechocystis sp to 24% for the minimal genome of Mycoplasma genitalium, and 23% in Thermotoga maritima. The coverage of the archaeal genomes was only slightly lower than that of bacterial genomes. The majority of the conserved gene strings are known operons, with the ribosomal superoperon being the top-scoring string in most genome comparisons. However, in some of the bacterial-archaeal pairs, the superoperon is rearranged to the extent that other operons, primarily those subject to horizontal transfer, show the greatest level of conservation, such as the archaeal-type H+-ATPase operon or ABC-type transport cassettes. The level of gene order conservation among prokaryotic genomes was compared to the cooccurrence of genomes in clusters of orthologous genes (COGs) and to the conservation of protein sequences themselves. Only limited correlation was observed between these evolutionary variables. Gene order conservation shows a much lower variance than the cooccurrence of genomes in COGs, which indicates that intragenome homogenization via recombination occurs in evolution much faster than intergenome homogenization via horizontal gene transfer and lineage-specific gene loss. The potential of using template-anchored multiple-genome alignments for predicting functions of uncharacterized genes was quantitatively assessed. Functions were predicted or significantly clarified for approximately 90 COGs (approximately 4% of the total of 2414 analyzed COGs). The most significant predictions were obtained for the poorly characterized archaeal genomes; these include a previously uncharacterized restriction-modification system, a nuclease-helicase combination implicated in DNA repair, and the probable archaeal counterpart of the eukaryotic exosome. Multiple genome alignments are a resource for studies on operon rearrangement and disruption, which is central to our understanding of the evolution of prokaryotic genomes. Because of the rapid evolution of the gene order, the potential of genome alignment for prediction of gene functions is limited, but nevertheless, such predictions information significantly complements the results obtained through protein sequence and structure analysis. (+info)
Identification of thermophilic species by the amino acid compositions deduced from their genomes.
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The global amino acid compositions as deduced from the complete genomic sequences of six thermophilic archaea, two thermophilic bacteria, 17 mesophilic bacteria and two eukaryotic species were analysed by hierarchical clustering and principal components analysis. Both methods showed an influence of several factors on amino acid composition. Although GC content has a dominant effect, thermophilic species can be identified by their global amino acid compositions alone. This study presents a careful statistical analysis of factors that affect amino acid composition and also yielded specific features of the average amino acid composition of thermophilic species. Moreover, we introduce the first example of a 'compositional tree' of species that takes into account not only homologous proteins, but also proteins unique to particular species. We expect this simple yet novel approach to be a useful additional tool for the study of phylogeny at the genome level. (+info)
Lineage-specific gene expansions in bacterial and archaeal genomes.
(30/588)
Gene duplication is an important mechanistic antecedent to the evolution of new genes and novel biochemical functions. In an attempt to assess the contribution of gene duplication to genome evolution in archaea and bacteria, clusters of related genes that appear to have expanded subsequent to the diversification of the major prokaryotic lineages (lineage-specific expansions) were analyzed. Analysis of 21 completely sequenced prokaryotic genomes shows that lineage-specific expansions comprise a substantial fraction (approximately 5%-33%) of their coding capacities. A positive correlation exists between the fraction of the genes taken up by lineage-specific expansions and the total number of genes in a genome. Consistent with the notion that lineage-specific expansions are made up of relatively recently duplicated genes, >90% of the detected clusters consists of only two to four genes. The more common smaller clusters tend to include genes with higher pairwise similarity (as reflected by average score density) than larger clusters. Regardless of size, cluster members tend to be located more closely on bacterial chromosomes than expected by chance, which could reflect a history of tandem gene duplication. In addition to the small clusters, almost all genomes also contain rare large clusters of size > or =20. Several examples of the potential adaptive significance of these large clusters are explored. The presence or absence of clusters and their related genes was used as the basis for the construction of a similarity graph for completely sequenced prokaryotic genomes. The topology of the resulting graph seems to reflect a combined effect of common ancestry, horizontal transfer, and lineage-specific gene loss. (+info)
Analysis of codon usage patterns of bacterial genomes using the self-organizing map.
(31/588)
Codon usage varies both between organisms and between different genes in the same organism. This observation has been used as a basis for earlier work in identifying highly expressed and horizontally transferred genes in Escherichia coli. In this work, we applied Kohonen's self-organizing map to analysis of the codon usage pattern of the Escherichia coli, Aquifex aeolicus, Archaeoglobus fulgidus, Haemophilus influenzae RD:, Methanococcus jannaschii, Methanobacterium thermoautotrophicum, and Pyrococcus horikoshii genomes for evidence of highly expressed genes and horizontally transferred genes. All of the analyzed genomes had a clear category of horizontally transferred genes, and their apparent percentages ranged from 7.7% to 21.4%. The apparent percentage of highly expressed genes ranges from 0% to 11.8%. A clustering of average codon usage of main gene categories of the seven genomes showed an interesting mixing of gene classes in four thermophilic/hyperthermophilic organisms, A. aeolicus, A. fulgidus, M. thermoautotrophicum, and P. horikoshii, which suggests possible origins of their horizontally transferred genes as well as the need for adaptation to a specific environment. Further classification of the three gene categories in E. coli and H. influenzae according to gene function revealed that genes involved in communication (such as regulation and cell process) and structure (cell structure and structural proteins) are more likely to be horizontally transferred than are genes involved in information (transcription, translation, and related processes) and in some groups of energy (such as energy metabolism and carbon compound catabolism). (+info)
Genomic analysis of the histidine kinase family in bacteria and archaea.
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Two-component signal transduction systems, consisting of histidine kinase (HK) sensors and DNA-binding response regulators, allow bacteria and archaea to respond to diverse environmental stimuli. HKs possess a conserved domain (H-box region) which contains the site of phosphorylation and an ATP-binding kinase domain. In this study, a genomic approach was taken to analyse the HK family in bacteria and archaea. Based on phylogenetic analysis, differences in the sequence and organization of the H-box and kinase domains, and the predicted secondary structure of the H-box region, five major HK types were identified. Of the 336 HKs analysed, 92% could be assigned to one of the five major HK types. The Type I HKs were found predominantly in bacteria while Type II HKs were not prevalent in bacteria but constituted the major type (13 of 15 HKs) in the archaeon Archaeoglobus fulgidus. Type III HKs were generally more prevalent in Gram-positive bacteria and were the major HK type (14 of 15 HKs) in the archaeon Methanobacterium thermoautotrophicum. Type IV HKs represented a minor type found in bacteria. The fifth HK type was composed of the chemosensor HKs, CheA. Several bacterial genomes contained all five HK types. In contrast, archaeal genomes either contained a specific HK type or lacked HKs altogether. These findings suggest that the different HK types originated in bacteria and that specific HK types were acquired in archaea by horizontal gene transfer. (+info)