Genetic exchange mediated by viruses of bacteria (bacteriophages) is the primary driver of rapid bacterial evolution. The priority of viruses is usually to propagate themselves. Most bacteriophages use the small terminase protein to identify their own genome and direct its inclusion into phage capsids. Gene transfer agents (GTAs) are descended from bacteriophages, but they instead package fragments of the entire bacterial genome without preference for their own genes. GTAs do not selectively target specific DNA, and no GTA small terminases are known. Here, we identified the small terminase from the model Rhodobacter capsulatus GTA, which then allowed prediction of analogues in other species. We examined the role of the small terminase in GTA production and propose a structural basis for random DNA packaging.IMPORTANCE Random transfer of any and all genes between bacteria could be influential in the spread of virulence or antimicrobial resistance genes. Discovery of the true prevalence of GTAs in sequenced genomes is hampered by their apparent similarity to bacteriophages. Our data allowed the prediction of small terminases in diverse GTA producer species, and defining the characteristics of a "GTA-type" terminase could be an important step toward novel GTA identification. Importantly, the GTA small terminase shares many features with its phage counterpart. We propose that the GTA terminase complex could become a streamlined model system to answer fundamental questions about double-stranded DNA (dsDNA) packaging by viruses that have not been forthcoming to date.

中文翻译：

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荚膜红细菌中第一个基因转移剂（GTA）小终止酶的鉴定及其在GTA生产和DNA包装中的作用。

由细菌（噬菌体）病毒介导的遗传交换是细菌快速进化的主要驱动力。病毒的优先级通常是传播自身。大多数噬菌体使用小末端酶蛋白来鉴定自己的基因组，并将其包含在噬菌体衣壳中。基因转移剂（GTA）来源于噬菌体，但它们包装整个细菌基因组的片段，而不偏爱它们自己的基因。GTA不能选择性地靶向特定的DNA，也没有GTA小末端酶。在这里，我们从荚膜红细菌GTA模型中鉴定了小末端酶，然后可以预测其他物种的类似物。我们检查了小末端酶在GTA生产中的作用，并提出了随机DNA包装的结构基础。重要信息细菌之间任何和所有基因的随机转移都可能影响毒力或抗菌素耐药基因的传播。它们与噬菌体的明显相似性阻碍了测序基因组中GTAs真正流行的发现。我们的数据可以预测各种GTA生产者物种中的小末端，并且定义“ GTA型”末端酶的特征可能是朝着新颖的GTA鉴定迈出的重要一步。重要的是，GTA小末端酶与其噬菌体对应物具有许多功能。我们建议，GTA末端酶复合物可以成为简化的模型系统，以回答关于迄今为止尚未出现的病毒对双链DNA（dsDNA）包装的基本问题。