Selection for reducing energy cost of protein production drives the GC content and amino acid composition bias in gene transfer agents
Gene transfer agents (GTAs) are virus-like elements integrated into bacterial genomes, particularly, those of Alphaproteobacteria. The GTAs can be induced under nutritional stress, incorporate random fragments of bacterial DNA into mini-phage particles, lyse the host cells and infect neighboring bacteria, thus enhancing horizontal gene transfer. We show that the GTA genes evolve under pronounced positive selection for the reduction of the energy cost of protein production as shown by comparison of the amino acid compositions with both homologous viral genes and host genes. The energy saving in GTA genes is comparable to or even more pronounced than that in the genes encoding the most abundant, essential bacterial proteins. In cases when viruses acquire genes from GTAs, the bias in amino acid composition disappears in the course of evolution, showing that reduction of the energy cost of protein is an important factor of evolution of GTAs but not bacterial viruses. These findings strongly suggest that GTAs are bacterial adaptations rather than selfish, virus-like elements. Because GTA production kills the host cell and does not propagate the GTA genome, it appears likely that the GTAs are retained in the course of evolution via kin or group selection. Therefore, we hypothesize that GTA facilitate the survival of bacterial populations under energy-limiting conditions through the spread of metabolic and transport capabilities via horizontal gene transfer and increase of nutrient availability resulting from the altruistic suicide of GTA-producing cells. ImportanceKin and group selection remain controversial topics in evolutionary biology. We argue that these types of selection are likely to operate in bacterial populations by showing that bacterial Gene Transfer Agents (GTAs), but not related viruses, evolve under positive selection for the reduction of the energy cost of a GTA particle production. We hypothesize that GTAs are dedicated devices for the survival of bacteria under the conditions of nutrient limitation. The benefits conferred by GTAs under nutritional stress appear to include horizontal dissemination of genes that could provide bacteria with enhanced capabilities for nutrient utilization and the increase of nutrient availability through the lysis of GTA-producing bacteria.