Gene transfer agents (GTAs) are virus-like elements integrated into bacterial genomes, particularly, those of Alphaproteobacteria. The GTAs can be induced under conditions of nutritional stress, incorporate random fragments of bacterial DNA into miniphage particles, lyse the host cells, and infect neighboring bacteria, thus enhancing horizontal gene transfer. We show that GTA genes evolve under conditions of pronounced positive selection for the reduction of the energy cost of protein production as shown by comparison of the amino acid compositions with those of 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 in which 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 production is an important factor of evolution of GTAs but not bacterial viruses. These findings strongly suggest that GTAs represent 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 GTAs facilitate the survival of bacterial populations under energy-limiting conditions through the spread of metabolic and transport capabilities via horizontal gene transfer and increases in nutrient availability resulting from the altruistic suicide of GTA-producing cells.

中文翻译：

---

降低蛋白质生产能源成本的选择推动了基因转移剂中GC含量和氨基酸组成的偏向。

基因转移剂（GTA）是整合到细菌基因组中的病毒样成分，特别是Alphaproteobacteria的那些。可以在营养压力条件下诱导GTAs，将细菌DNA的随机片段掺入微噬菌体颗粒中，裂解宿主细胞，并感染邻近细菌，从而增强水平基因转移。我们显示，GTA基因在显着阳性选择的条件下进化，可降低蛋白质生产的能源成本，如氨基酸组成与同源病毒基因和宿主基因的氨基酸组成的比较所示。与编码最丰富，必需的细菌蛋白的基因相比，GTA基因的节能效果可比甚至更加明显。如果病毒从GTA获取基因，则氨基酸组成的偏差会在进化过程中消失，表明降低蛋白质生产的能源成本是GTAs进化的重要因素，而不是细菌病毒。这些发现强烈表明，GTA代表细菌适应而不是自私的病毒样元素。由于GTA的产生会杀死宿主细胞并且不会繁殖GTA基因组，因此GTA可能会通过亲属或群体选择保留在进化过程中。因此，我们假设GTA通过水平基因转移通过新陈代谢和转运功能的传播，促进了能量限制条件下细菌种群的生存，并且由于生产GTA的利他性自杀导致养分利用率提高。这些发现强烈表明，GTA代表细菌适应而不是自私的病毒样元素。由于GTA的产生会杀死宿主细胞并且不会繁殖GTA基因组，因此GTA可能会通过亲属或群体选择保留在进化过程中。因此，我们假设GTA通过水平基因转移通过新陈代谢和转运功能的传播，促进了能量限制条件下细菌种群的生存，并且由于生产GTA的利他性自杀导致养分利用率提高。这些发现强烈表明，GTA代表细菌适应而不是自私的病毒样元素。由于GTA的产生会杀死宿主细胞并且不会繁殖GTA基因组，因此GTA可能会通过亲属或群体选择保留在进化过程中。因此，我们假设GTA通过水平基因转移通过新陈代谢和转运功能的传播，促进了能量限制条件下细菌种群的生存，并且由于生产GTA的利他性自杀导致养分利用率提高。