RibAlign: a software tool and database for eubacterial phylogeny based on concatenated ribosomal protein subunits

Conservation and Evolution of the Sporulation Gene Set in Diverse Members of the Firmicutes

The current classification of the phylum Firmicutes (new name, Bacillota) features eight distinct classes, six of which include known spore-forming bacteria. In Bacillus subtilis, sporulation involves up to 500 genes, many of which do not have orthologs in other bacilli and/or clostridia. Previous studies identified about 60 sporulation genes of B. subtilis that were shared by all spore-forming members of the Firmicutes. These genes are referred to as the sporulation core or signature, although many of these are also found in genomes of nonsporeformers. Using an expanded set of 180 firmicute genomes from 160 genera, including 76 spore-forming species, we investigated the conservation of the sporulation genes, in particular seeking to identify lineages that lack some of the genes from the conserved sporulation core. The results of this analysis confirmed that many small acid-soluble spore proteins (SASPs), spore coat proteins, and germination proteins, which were previously characterized in bacilli, are missing in spore-forming members of Clostridia and other classes of Firmicutes. A particularly dramatic loss of sporulation genes was observed in the spore-forming members of the families Planococcaceae and Erysipelotrichaceae. Fifteen species from diverse lineages were found to carry skin (sigK-interrupting) elements of different sizes that all encoded SpoIVCA-like recombinases but did not share any other genes. Phylogenetic trees built from concatenated alignments of sporulation proteins and ribosomal proteins showed similar topology, indicating an early origin and subsequent vertical inheritance of the sporulation genes. IMPORTANCE Many members of the phylum Firmicutes (Bacillota) are capable of producing endospores, which enhance the survival of important Gram-positive pathogens that cause such diseases as anthrax, botulism, colitis, gas gangrene, and tetanus. We show that the core set of sporulation genes, defined previously through genome comparisons of several bacilli and clostridia, is conserved in a wide variety of sporeformers from several distinct lineages of Firmicutes. We also detected widespread loss of sporulation genes in many organisms, particularly within the families Planococcaceae and Erysipelotrichaceae. Members of these families, such as Lysinibacillus sphaericus and Clostridium innocuum, could be excellent model organisms for studying sporulation mechanisms, such as engulfment, formation of the spore coat, and spore germination.

Revising the Structural Diversity of Ribosomal Proteins Across the Three Domains of Life

Ribosomal proteins are indispensable components of a living cell, and yet their structures are remarkably diverse in different species. Here we use manually curated structural alignments to provide a comprehensive catalog of structural variations in homologous ribosomal proteins from bacteria, archaea, eukaryotes, and eukaryotic organelles. By resolving numerous ambiguities and errors of automated structural and sequence alignments, we uncover a whole new class of structural variations that reside within seemingly conserved segments of ribosomal proteins. We then illustrate that these variations reflect an apparent adaptation of ribosomal proteins to the specific environments and lifestyles of living species. Finally, we show that most of these structural variations reside within nonglobular extensions of ribosomal proteins-protein segments that are thought to promote ribosome biogenesis by stabilizing the proper folding of ribosomal RNA. We show that although the extensions are thought to be the most ancient peptides on our planet, they are in fact the most rapidly evolving and most structurally and functionally diverse segments of ribosomal proteins. Overall, our work illustrates that, despite being long considered as slowly evolving and highly conserved, ribosomal proteins are more complex and more specialized than is generally recognized.

RiboDB Database: A Comprehensive Resource for Prokaryotic Systematics

Ribosomal proteins (r-proteins) are increasingly used as an alternative to ribosomal rRNA for prokaryotic systematics. However, their routine use is difficult because r-proteins are often not or wrongly annotated in complete genome sequences, and there is currently no dedicated exhaustive database of r-proteins. RiboDB aims at fulfilling this gap. This weekly updated comprehensive database allows the fast and easy retrieval of r-protein sequences from publicly available complete prokaryotic genome sequences. The current version of RiboDB contains 90 r-proteins from 3,750 prokaryotic complete genomes encompassing 38 phyla/major classes and 1,759 different species. RiboDB is accessible at http://ribodb.univ-lyon1.fr and through ACNUC interfaces.

Phylogeny of a relapsing fever Borrelia species transmitted by the hard tick Ixodes scapularis

The discovery of Borrelia species that were related to the agents of relapsing fever but were transmitted by hard ticks rather than soft ticks challenged previous taxonomies based largely on microbe-host specificities and geographic considerations. One of these newly-identified organisms is the Borrelia miyamotoi sensu lato strain LB-2001 from North America and transmitted by Ixodes scapularis. This or related strains have been identified as the cause of human disease, but comparatively little is known about their biology or genetics. Using recently acquired chromosome sequence of LB-2001 together with database sequences and additional sequences determined here, I carried out comparisons of the several species of Borrelia, including those in the two major clades: the relapsing fever group of species and the Lyme disease group of species. Phylogenetic inference at the species level was based on four data sets: whole chromosomes of ∼1Mb each, and concatenated sequences of 19 ribosomal protein genes, 3 conserved nucleic acid enzymes (rpoC, recC, and dnaE), and 4 contiguous genes for nucleotide salvage on a large plasmid. Analyses using neighbor-joining, maximum likelihood, and Bayesian methods were largely concordant for each of the trees. They showed that LB-2001 and related hard tick-associated organisms, like Borrelia lonestari, are deeply positioned within the RF group of species and that these organisms did not, as some earlier estimations had suggested, constitute a paraphyletic group. The analyses also provided further evidence that major changes in host ranges and life cycles, such as hard to soft ticks or vice versa, may not correlate well with overall sequence differences. The genetic differences between LB-2001 and B. miyamotoi sensu stricto justify provisional use of the "sensu lato" designation for LB-2001.

Phylogenomics of prokaryotic ribosomal proteins

Archaeal and bacterial ribosomes contain more than 50 proteins, including 34 that are universally conserved in the three domains of cellular life (bacteria, archaea, and eukaryotes). Despite the high sequence conservation, annotation of ribosomal (r-) protein genes is often difficult because of their short lengths and biased sequence composition. We developed an automated computational pipeline for identification of r-protein genes and applied it to 995 completely sequenced bacterial and 87 archaeal genomes available in the RefSeq database. The pipeline employs curated seed alignments of r-proteins to run position-specific scoring matrix (PSSM)-based BLAST searches against six-frame genome translations, mitigating possible gene annotation errors. As a result of this analysis, we performed a census of prokaryotic r-protein complements, enumerated missing and paralogous r-proteins, and analyzed the distributions of ribosomal protein genes among chromosomal partitions. Phyletic patterns of bacterial and archaeal r-protein genes were mapped to phylogenetic trees reconstructed from concatenated alignments of r-proteins to reveal the history of likely multiple independent gains and losses. These alignments, available for download, can be used as search profiles to improve genome annotation of r-proteins and for further comparative genomics studies.

Ribosomal multilocus sequence typing: universal characterization of bacteria from domain to strain

No single genealogical reconstruction or typing method currently encompasses all levels of bacterial diversity, from domain to strain. We propose ribosomal multilocus sequence typing (rMLST), an approach which indexes variation of the 53 genes encoding the bacterial ribosome protein subunits (rps genes), as a means of integrating microbial genealogy and typing. As with multilocus sequence typing (MLST), rMLST employs curated reference sequences to identify gene variants efficiently and rapidly. The rps loci are ideal targets for a universal characterization scheme as they are: (i) present in all bacteria; (ii) distributed around the chromosome; and (iii) encode proteins which are under stabilizing selection for functional conservation. Collectively, the rps loci exhibit variation that resolves bacteria into groups at all taxonomic and most typing levels, providing significantly more resolution than 16S small subunit rRNA gene phylogenies. A web-accessible expandable database, comprising whole-genome data from more than 1900 bacterial isolates, including 28 draft genomes assembled de novo from the European Bioinformatics Institute (EBI) sequence read archive, has been assembled. The rps gene variation catalogued in this database permits rapid and computationally non-intensive identification of the phylogenetic position of any bacterial sequence at the domain, phylum, class, order, family, genus, species and strain levels. The groupings generated with rMLST data are consistent with current nomenclature schemes and independent of the clustering algorithm used. This approach is applicable to the other domains of life, potentially providing a rational and universal approach to the classification of life that is based on one of its fundamental features, the translation mechanism.

Fructose 1,6-bisphosphate aldolase/phosphatase may be an ancestral gluconeogenic enzyme

Most archaeal groups and deeply branching bacterial lineages harbour thermophilic organisms with a chemolithoautotrophic metabolism. They live at high temperatures in volcanic habitats at the expense of inorganic substances, often under anoxic conditions. These autotrophic organisms use diverse carbon dioxide fixation mechanisms generating acetyl-coenzyme A, from which gluconeogenesis must start. Here we show that virtually all archaeal groups as well as the deeply branching bacterial lineages contain a bifunctional fructose 1,6-bisphosphate (FBP) aldolase/phosphatase with both FBP aldolase and FBP phosphatase activity. This enzyme is missing in most other Bacteria and in Eukaryota, and is heat-stabile even in mesophilic marine Crenarchaeota. Its bifunctionality ensures that heat-labile triosephosphates are quickly removed and trapped in stabile fructose 6-phosphate, rendering gluconeogenesis unidirectional. We propose that this highly conserved, heat-stabile and bifunctional FBP aldolase/phosphatase represents the pace-making ancestral gluconeogenic enzyme, and that in evolution gluconeogenesis preceded glycolysis.

Translational machinery of senegalese sole (Solea senegalensis Kaup) and Atlantic halibut (Hippoglossus hippoglossus L.): comparative sequence analysis of the complete set of 60s ribosomal proteins and their expression

Ribosomal proteins (RPs) comprise a large set of highly evolutionarily conserved proteins that are often over-represented in complementary DNA libraries. They have become very useful markers in comparative genomics, genome evolution, and phylogenetic studies across taxa. In this study, we report the sequences of the complete set of 60S RPs in Senegalese sole (Solea senegalensis) and Atlantic halibut (Hippoglossus hippoglossus), two commercially important flatfish species. Amino-acid sequence comparisons of the encoded proteins showed a high similarity both between these two flatfish species and with respect to other fish and human counterparts. Expressed sequence tag analysis revealed the existence of paralogous genes for RPL3, RPL7, RPL41, and RPLP2 in Atlantic halibut and RPL13a in Senegalese sole as well as RPL19 and RPL22 in both species. Phylogenetic analysis of paralogs revealed distinct evolutionary histories for each RP in agreement with three rounds of genome duplications and lineage-specific duplications during flatfish evolution. Steady-state transcript levels for RPL19 and RPL22 RPs were quantitated during larval development and in different tissues of sole and halibut using a real-time polymerase chain reaction approach. All paralogs were expressed ubiquitously although at different levels in different tissues. Most RP transcripts increased coordinately after larval first-feeding in both species but decreased progressively during the metamorphic process. In all cases, expression profiles and transcript levels of orthologous genes in Senegalese sole and Atlantic halibut were highly congruent. The genomic resources and knowledge developed in this survey will be useful for the study of Pleuronectiformes evolution.

Evolution and functional characterization of the RH50 gene from the ammonia-oxidizing bacterium Nitrosomonas europaea

The family of ammonia and ammonium channel proteins comprises the Amt proteins, which are present in all three domains of life with the notable exception of vertebrates, and the homologous Rh proteins (Rh50 and Rh30) that have been described thus far only in eukaryotes. The existence of an RH50 gene in bacteria was first revealed by the genome sequencing of the ammonia-oxidizing bacterium Nitrosomonas europaea. Here we have used a phylogenetic approach to study the evolution of the N. europaea RH50 gene, and we show that this gene, probably as a component of an integron cassette, has been transferred to the N. europaea genome by horizontal gene transfer. In addition, by functionally characterizing the Rh50(Ne) protein and the corresponding knockout mutant, we determined that NeRh50 can mediate ammonium uptake. The RH50(Ne) gene may thus have replaced functionally the AMT gene, which is missing in the genome of N. europaea and may be regarded as a case of nonorthologous gene displacement.

Function of heterologous and truncated RNase P proteins in Bacillus subtilis

Bacterial RNase P is composed of an RNA subunit and a single protein (encoded by the rnpB and rnpA genes respectively). The Bacillus subtilis rnpA knockdown strain d7 was used to screen for functional conservation among bacterial RNase P proteins from a representative spectrum of bacterial subphyla. We demonstrate conserved function of bacterial RNase P (RnpA) proteins despite low sequence conservation. Even rnpA genes from psychrophilic and thermophilic bacteria rescued growth of B. subtilis d7 bacteria; likewise, terminal extensions and insertions between beta strands 2 and 3, in the so-called metal binding loop, were compatible with RnpA function in B. subtilis. A deletion analysis of B. subtilis RnpA defined the structural elements essential for bacterial RNase P function in vivo. We further extended our complementation analysis in B. subtilis strain d7 to the four individual RNase P protein subunits from three different Archaea, as well as to human Rpp21 and Rpp29 as representatives of eukaryal RNase P. None of these non-bacterial RNase P proteins showed any evidence of being able to replace the B. subtilis RNase P protein in vivo, supporting the notion that archaeal/eukaryal RNase P proteins are evolutionary unrelated to the bacterial RnpA protein.

Proteorhodopsin photosystem gene clusters exhibit co-evolutionary trends and shared ancestry among diverse marine microbial phyla

Since the recent discovery of retinylidene proteins in marine bacteria (proteorhodopsins), the estimated abundance and diversity of this gene family has expanded rapidly. To explore proteorhodopsin photosystem evolutionary and distributional trends, we identified and compared 16 different proteorhodopsin-containing genome fragments recovered from naturally occurring bacterioplankton populations. In addition to finding several deep-branching proteorhodopsin sequences, proteorhodopsins were found in novel taxonomic contexts, including a betaproteobacterium and a planctomycete. Approximately one-third of the proteorhodopsin-containing genome fragments analysed, as well as a number of recently reported marine bacterial whole genome sequences, contained a linked set of genes required for biosynthesis of the rhodopsin chromophore, retinal. Phylogenetic analyses of the retinal biosynthetic genes suggested their co-evolution and probable coordinated lateral gene transfer into disparate lineages, including Euryarchaeota, Planctomycetales, and three different proteobacterial lineages. The lateral transfer and retention of genes required to assemble a functional proteorhodopsin photosystem appears to be a coordinated and relatively frequent evolutionary event. Strong selection pressure apparently acts to preserve these light-dependent photosystems in diverse marine microbial lineages.

Targeted access to the genomes of low-abundance organisms in complex microbial communities

Current metagenomic approaches to the study of complex microbial consortia provide a glimpse into the community metabolism and occasionally allow genomic assemblies for the most abundant organisms. However, little information is gained for the members of the community present at low frequencies, especially those representing yet-uncultured taxa, which include the bulk of the diversity present in most environments. Here we used phylogenetically directed cell separation by fluorescence in situ hybridization and flow cytometry, followed by amplification and sequencing of a fraction of the genomic DNA of several bacterial cells that belong to the TM7 phylum. Partial genomic assembly allowed, for the first time, a look into the evolution and potential metabolism of a soil representative from this group of organisms for which there are no species in stable laboratory cultures. Genomic reconstruction from targeted cells of uncultured organisms isolated directly from the environment represents a powerful approach to access any specific members of a community and an alternative way to assess the community's metabolic potential.

Plasticity of the domain structure in FlgJ, a bacterial protein involved in flagellar rod formation

Bacterial flagellar rod structure is built across the peptidoglycan (PG) layer. A Salmonella enterica flagellar protein FlgJ is believed to consist of two functional domains, the N-terminal half acting as a scaffold or cap essential for rod assembly and the C-terminal half acting as a PG hydrolase (PGase) that makes a hole in the PG layer to facilitate rod penetration. In this study, molecular data analyses were conducted on FlgJ data sets sampled from a variety of bacterial species, and three types of FlgJ homologs were identified: (i) "canonical dual-domain" type found in beta- and gamma-proteobacteria that has a domain for one of the PGases, acetylmuramidase (Acm), at the C terminus, (ii) "non-canonical dual-domain" type found in the genus Desulfovibrio (delta-proteobacteria) that bears a domain for another PGase, M23/M37-family peptidase (Pep), at the C terminus and (iii) "single-domain" type found in phylogenetically diverged lineages that lacks the Acm or Pep domain. FlgJ phylogeny, together with the domain architecture, suggested that the single-domain type was the original form of FlgJ and the canonical dual-domain type had evolved from the single-domain type by fusion of the Acm domain to its C terminus in the common ancestor of beta- and gamma-proteobacteria. The non-canonical dual-domain type may have been formed by fusion of the Pep domain to the single-domain type in the ancestor of Desulfovibrio. In some lineages of gamma-proteobacteria, the Acm domain appeared to be lost secondarily from the dual-domain type FlgJ to yield again a single-domain type one. To rationalize the underlying mechanism that gave rise to the two different types of dual-domain FlgJ homologs, we propose a model assuming the lineage-specific co-option of flagellum-specific PGase from diverged housekeeping PGases in bacteria.