Gene transfer agents (GTAs) are genetic elements derived from ancestral bacteriophages that have become domesticated by the host. GTAs are present in diverse prokaryotic organisms, where they can facilitate horizontal gene transfer under certain conditions. Unlike typical bacteriophages, GTAs do not exhibit any preference for the replication or transfer of the genes encoding them; instead, they exhibit a remarkable capacity to package chromosomal, and sometimes extrachromosomal, DNA into virus‐like capsids and disseminate it to neighboring cells. Because GTAs resemble defective prophages, identification of novel GTAs is not trivial. The detection of candidates relies on the genetic similarity to known GTAs, which has been fruitful in α‐proteobacterial lineages but challenging in more distant bacteria. Here we consider several fundamental questions: What is the true prevalence of GTAs in prokaryote genomes? Given there are high costs for GTA production, what advantage do GTAs provide to the bacterial host to justify their maintenance? How is the bacterial chromosome recognized and processed for inclusion in GTA particles? This article highlights the challenges in comprehensively understanding GTAs' prevalence, function and DNA packaging method. Going forward, broad study of atypical GTAs and use of ecologically relevant conditions are required to uncover their true impact on bacterial chromosome evolution.

中文翻译：

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基因转移剂：无私病毒在基因交换和细菌进化中的模糊作用

基因转移剂（GTA）是源自已被宿主驯化的祖先噬菌体的遗传元件。 GTA 存在于多种原核生物中，在某些条件下可以促进水平基因转移。与典型的噬菌体不同，GTA 对编码它们的基因的复制或转移没有表现出任何偏好；相反，它们表现出惊人的能力，可以将染色体（有时是染色体外）DNA 包装到病毒样衣壳中，并将其传播到邻近的细胞。由于 GTA 类似于有缺陷的前噬菌体，因此新型 GTA 的鉴定并非易事。候选物的检测依赖于与已知 GTA 的遗传相似性，这在 α-变形菌谱系中取得了丰硕成果，但在更遥远的细菌中具有挑战性。在这里，我们考虑几个基本问​​题：原核生物基因组中 GTA 的真实流行程度是多少？鉴于 GTA 生产成本高昂，GTA 为细菌宿主提供了什么优势来证明其维护的合理性？细菌染色体如何被识别和加工以包含在 GTA 颗粒中？本文重点介绍了全面了解 GTA 的流行、功能和 DNA 包装方法所面临的挑战。展望未来，需要对非典型 GTA 进行广泛研究并利用生态相关条件，以揭示它们对细菌染色体进化的真正影响。