Ectopic gene conversions in bacterial genomes

Rates of gene conversions between Escherichia coli ribosomal operons

Due to their universal presence and high sequence conservation, ribosomal RNA (rRNA) sequences are used widely in phylogenetics for inferring evolutionary relationships between microbes and in metagenomics for analyzing the composition of microbial communities. Most microbial genomes encode multiple copies of rRNA genes to supply cells with sufficient capacity for protein synthesis. These copies typically undergo concerted evolution that keeps their sequences identical, or nearly so, due to gene conversion, a type of intragenomic recombination that changes one copy of a homologous sequence to exactly match another. Widely varying rates of rRNA gene conversion have previously been estimated by comparative genomics methods and using genetic reporter assays. To more directly measure rates of rRNA intragenomic recombination, we sequenced the seven Escherichia coli rRNA operons in 15 lineages that were evolved for ∼13,750 generations with frequent single-cell bottlenecks that reduce the effects of selection. We identified 38 gene conversion events and estimated an overall rate of intragenomic recombination within the 16S and 23S genes between rRNA copies of 3.6 × 10⁻⁴ per genome per generation or 8.6 × 10⁻⁶ per rRNA operon per homologous donor operon per generation. This rate varied only slightly from random expectations at different sites within the rRNA genes and between rRNA operons located at different positions in the genome. Our accurate estimate of the rate of rRNA gene conversions fills a gap in our quantitative understanding of how ribosomal sequences and other multicopy elements diversify and homogenize during microbial genome evolution.

Bacterial genome instability

Bacterial genomes are remarkably stable from one generation to the next but are plastic on an evolutionary time scale, substantially shaped by horizontal gene transfer, genome rearrangement, and the activities of mobile DNA elements. This implies the existence of a delicate balance between the maintenance of genome stability and the tolerance of genome instability. In this review, we describe the specialized genetic elements and the endogenous processes that contribute to genome instability. We then discuss the consequences of genome instability at the physiological level, where cells have harnessed instability to mediate phase and antigenic variation, and at the evolutionary level, where horizontal gene transfer has played an important role. Indeed, this ability to share DNA sequences has played a major part in the evolution of life on Earth. The evolutionary plasticity of bacterial genomes, coupled with the vast numbers of bacteria on the planet, substantially limits our ability to control disease.

Strong purifying selection against gene conversions in the trypsin genes of primates

The trypsin gene families of primate species are composed of members who share a remarkable level of sequence similarity. Here, we investigated the gene conversions occurring within the trypsin gene family in five primate species. A total of 36 conversion events, with an average length (±standard deviation) of 1,526 ± 1,124 nucleotides, were detected using two methods. Such extensive gene conversions are likely both the cause and the consequence of the high sequence similarity between primate trypsin genes. In the trypsins encoded by these genes, both the overall amino acid sequences and critical amino acid residues are conserved. Therefore, the numerous long gene conversions we detected between trypsin genes did not alter any of their functionally important amino acid sites. This suggest that, in the trypsin genes of the five primate species studied here, strong purifying selection against gene conversions is occurring in regions containing functionally important residues.

Gene conversions in the growth hormone gene family of primates: stronger homogenizing effects in the Hominidae lineage

In humans, the growth hormone/chorionic somatomammotropin gene family is composed of five highly similar genes. We characterized the gene conversions that occurred between the growth hormone genes of 11 primate species. We detected 48 conversions using GENECONV and others were only detected using phylogenetic analyses. Gene conversions were detected in all species analyzed, their average size (±standard deviation) is 197.8±230.4 nucleotides, the size of the conversions is correlated with sequence similarity and converted regions are significantly more GC-rich than non-converted regions. Gene conversions have a stronger homogenizing effect in Hominidae genes than in other primate species. They are also less frequent in conserved gene regions and towards functionally important genes. This suggests that the high degree of sequence similarity observed between the growth hormone genes of primate species is a consequence of frequent gene conversions in gene regions which are under little selective constraints.

Lateral transfer of genes and gene fragments in Staphylococcus extends beyond mobile elements

The widespread presence of antibiotic resistance and virulence among Staphylococcus isolates has been attributed in part to lateral genetic transfer (LGT), but little is known about the broader extent of LGT within this genus. Here we report the first systematic study of the modularity of genetic transfer among 13 Staphylococcus genomes covering four distinct named species. Using a topology-based phylogenetic approach, we found, among 1,354 sets of homologous genes examined, strong evidence of LGT in 368 (27.1%) gene sets, and weaker evidence in another 259 (19.1%). Within-gene and whole-gene transfer contribute almost equally to the topological discordance of these gene sets against a reference phylogeny. Comparing genetic transfer in single-copy and in multicopy gene sets, we observed a higher frequency of LGT in the latter, and a substantial functional bias in cases of whole-gene transfer (little such bias was observed in cases of fragmentary genetic transfer). We found evidence that lateral transfer, particularly of entire genes, impacts not only functions related to antibiotic, drug, and heavy-metal resistance, as well as membrane transport, but also core informational and metabolic functions not associated with mobile elements. Although patterns of sequence similarity support the cohesion of recognized species, LGT within S. aureus appears frequently to disrupt clonal complexes. Our results demonstrate that LGT and gene duplication play important parts in functional innovation in staphylococcal genomes.

Gene conversion results in the equalization of genome copies in the polyploid haloarchaeon Haloferax volcanii

Haloferax volcanii is highly polyploid and contains about 20 copies of the major chromosome. A heterozygous strain was constructed that contained two different types of genomes: the leuB locus contained either the wild-type leuB gene or a leuB:trpA gene introduced by gene replacement. As the trpA locus is devoid of the wild-type trpA gene, growth in the absence of both amino acids is only possible when both types of genomes are simultaneously present, exemplifying gene redundancy and the potential to form heterozygous cells as one possible evolutionary advantage of polyploidy. The heterozygous strain was grown (i) in the presence of tryptophan, selecting for the presence of leuB, (ii) in the presence of leucine selecting for leuB:trpA and (iii) in the absence of selection. Both types of genomes were quantified with real-time PCR. The first condition led to a complete loss of leuB:trpA-containing genomes, while under the second condition leuB-containing genomes were lost. Also in the absence of selection gene conversion led to a fast equalization of genomes and resulted in homozygous leuB-containing cells. Gene conversion leading to genome equalization can explain the escape from 'Muller's ratchet' as well as the ease of mutant construction using polyploid haloarchaea.

Gorgeous mosaic of mitochondrial genes created by horizontal transfer and gene conversion

The best known outcome of horizontal gene transfer (HGT) is the introduction of novel genes, but other outcomes have been described. When a transferred gene has a homolog in the recipient genome, the native gene may be functionally replaced (and subsequently lost) or partially overwritten by gene conversion with transiently present foreign DNA. Here we report the discovery, in two lineages of plant mitochondrial genes, of novel gene combinations that arose by conversion between coresident native and foreign homologs. These lineages have undergone intricate conversion between native and foreign copies, with conversion occurring repeatedly and differentially over the course of speciation, leading to radiations of mosaic genes involved in respiration and intron splicing. Based on these findings, we develop a model--the duplicative HGT and differential gene conversion model--that integrates HGT and ongoing gene conversion in the context of speciation. Finally, we show that one of these HGT-driven gene-conversional radiations followed two additional types of conversional chimerism, namely, intramitochondrial retroprocessing and interorganellar gene conversion across the 2 billion year divide between mitochondria and chloroplasts. These findings expand our appreciation of HGT and gene conversion as creative evolutionary forces, establish plant mitochondria as a premiere system for studying the evolutionary dynamics of HGT and its genetic reverberations, and recommend careful examination of bacterial and other genomes for similar, likely overlooked phenomena.

Identifying concerted evolution and gene conversion in mammalian gene pairs lasting over 100 million years

BACKGROUND

Concerted evolution occurs in multigene families and is characterized by stretches of homogeneity and higher sequence similarity between paralogues than between orthologues. Here we identify human gene pairs that have undergone concerted evolution, caused by ongoing gene conversion, since at least the human-mouse divergence. Our strategy involved the identification of duplicated genes with greater similarity within a species than between species. These genes were required to be present in multiple mammalian genomes, suggesting duplication early in mammalian divergence. To eliminate genes that have been conserved due to strong purifying selection, our analysis also required at least one intron to have retained high sequence similarity between paralogues.

RESULTS

We identified three human gene pairs undergoing concerted evolution (BMP8A/B, DDX19A/B, and TUBG1/2). Phylogenetic investigations reveal that in each case the duplication appears to have occurred prior to eutherian mammalian radiation, with exactly two paralogues present in all examined species. This indicates that all three gene duplication events were established over 100 million years ago.

CONCLUSION

The extended duration of concerted evolution in multiple distant lineages suggests that there has been prolonged homogenization of specific segments within these gene pairs. Although we speculate that selection for homogenization could have been utilized in order to maintain crucial homo- or hetero- binding domains, it remains unclear why gene conversion has persisted for such extended periods of time. Through these analyses, our results demonstrate additional examples of a process that plays a definite, although unspecified, role in molecular evolution.

The extent of migration of the Holliday junction is a crucial factor for gene conversion in Rhizobium etli

Gene conversion, defined as the nonreciprocal transfer of DNA, is one result of homologous recombination. Three steps in recombination could give rise to gene conversion: (i) DNA synthesis for repair of the degraded segment, (ii) Holliday junction migration, leading to heteroduplex formation, and (iii) repair of mismatches in the heteroduplex. There are at least three proteins (RuvAB, RecG, and RadA) that participate in the second step. Their roles have been studied for homologous recombination, but evidence of their relative role in gene conversion is lacking. In this work, we showed the effect on gene conversion of mutations in ruvB, recG, and radA in Rhizobium etli, either alone or in combination, using a cointegration strategy previously developed in our laboratory. The results indicate that the RuvAB system is highly efficient for gene conversion, since its absence provokes smaller gene conversion segments than those in the wild type as well as a shift in the preferred position of conversion tracts. The RecG system possesses a dual role for gene conversion. Inactivation of recG leads to longer gene conversion tracts than those in the wild type, indicating that its activity may hinder heteroduplex extension. However, under circumstances where it is the only migration activity present (as in the ruvB radA double mutant), conversion segments can still be seen, indicating that RecG can also promote gene conversion. RadA is the least efficient system in R. etli but is still needed for the production of detectable gene conversion tracts.

Gene conversion maintains nonfunctional transposable elements in an obligate mutualistic endosymbiont

Long-term bacterial endosymbionts typically exhibit reduced genomes, lack genes encoding recombination functions and transposable elements, such as insertion sequences (ISs). In sharp contrast, I found that ISs constitute 2.4% of the genome of the obligate mutualistic endosymbiont Wolbachia wBm. Although no IS copy is transpositionally functional, I show that ISs persist in wBm because of frequent recombinational gene conversion (GC) homogenizing homologous IS sequences. These results not only indicate that there exists a functional recombination molecular machinery in wBm, but they also suggest that, by slowing down the rate of IS degradation and loss, GC may represent a major force influencing reductive evolution in wBm.

Ectopic gene conversions in the human genome

We used the GENCONV method to characterize the gene conversions that occurred amongst the 1434 protein coding human gene families with three or more genes. Conversions occur at a frequency of 0.88% (483 conversion events/55,050 gene pairs compared) and have an average length of 371+/-752 bp (+/-standard deviation). Both the size and the frequency of conversions are positively correlated with the similarity of the sequences involved in these conversions. The frequency of conversions and the local recombination rate are also positively correlated. Intrachromosomal conversions are almost 5 times more frequent than interchromosomal conversions and the frequency of intrachromosomal conversions increases as the distance between genes decreases. However, the higher frequency of conversions between nearby genes with the same transcriptional orientation is due to the fact that most functional duplicated genes are found next to one another and in the same transcriptional orientation. The average length of a conversion spanning only an intron region is significantly smaller than conversions spanning both exons and introns or only exons. This suggests that the smaller degree of sequence similarity of introns limits the size of conversions between duplicated human genes. The significant excess of conversions at the 3'-end of human genes suggests that incomplete cDNA molecules are often involved in conversions with chromosomal gene copies.