Department of Biochemistry, University of Torontogrid.17063.33, Toronto, Ontario, Canada.
Department of Molecular Genetics, University of Torontogrid.17063.33, Toronto, Ontario, Canada.
mBio. 2022 Dec 20;13(6):e0217122. doi: 10.1128/mbio.02171-22. Epub 2022 Oct 31.
In bacteria, the mechanisms used to repair DNA lesions during genome replication include homologous recombination between sister chromosomes. This can lead to the formation of chromosome dimers if an odd number of crossover events occurs. The dimers must be resolved before cell separation to ensure genomic stability and cell viability. Dimer resolution is achieved by the broadly conserved /Xer system, which catalyzes one additional crossover event immediately prior to cell separation. While /Xer systems have been characterized or predicted in the vast majority of proteobacteria, no homologs to or have been identified in the order . Here, we report the discovery of a distinct single-recombinase /Xer system in the intracellular pathogen Legionella pneumophila. The site was uncovered by our analysis of mobile element-1 (LME-1), which harbors a site mimic and integrates into the L. pneumophila genome via site-specific recombination. We demonstrate that (here named ) encodes a tyrosine recombinase that is necessary and sufficient for catalyzing recombination at the site and that deletion of or causes filamentation along with extracellular and intracellular growth defects. We show that the XerL system is present throughout and that Coxiella burnetii XerL and its cognate site can functionally substitute for the native system in L. pneumophila. Finally, we describe an unexpected link between C. burnetii /Xer and the maintenance of its virulence plasmids. The maintenance of circular chromosomes depends on the ability to resolve aberrant chromosome dimers after they form. In most proteobacteria, broadly conserved Xer recombinases catalyze single crossovers at short, species-specific sites located near the replication terminus. Chromosomal dimerization leads to the formation of two copies of within the same molecule, leading to rapid site-specific recombination and conversion back into chromosome monomers. The apparent absence of chromosome dimer resolution mechanisms in has been a mystery to date. By studying a phage-like mobile genetic element, LME-1, we have identified a previously unknown single-recombinase /Xer system that is not only widespread across but whose activity is linked to virulence in two important human pathogens.
在细菌中，用于在基因组复制过程中修复 DNA 损伤的机制包括姐妹染色体之间的同源重组。如果发生奇数个交叉事件，可能会导致染色体二聚体的形成。在细胞分离之前，必须解决二聚体以确保基因组稳定性和细胞活力。二聚体的解决是通过广泛保守的 /Xer 系统实现的，该系统在细胞分离之前立即催化一个额外的交叉事件。虽然 /Xer 系统已在绝大多数变形菌中得到了描述或预测，但在 目 中尚未鉴定出 或 的同源物。在这里，我们报道了在细胞内病原体军团菌中发现的一种独特的单重组酶 /Xer 系统。通过对移动元件 1（LME-1）的分析发现了 位点，该元件含有 位点模拟物，并通过位点特异性重组整合到军团菌基因组中。我们证明了 （此处命名为 ）编码一种酪氨酸重组酶，该酶对于在 位点催化重组是必需和充分的，并且 或 的缺失会导致丝状化以及细胞外和细胞内生长缺陷。我们表明，XerL 系统存在于整个 中，并且 Coxiella burnetii XerL 和其同源 位点可以在 L. pneumophila 中替代天然系统发挥功能。最后，我们描述了 C. burnetii /Xer 与维持其毒力质粒之间的意外联系。圆形染色体的维持取决于形成后能够解决异常染色体二聚体的能力。在大多数变形菌中，广泛保守的 Xer 重组酶在短的、物种特异性的 位点催化单交叉，该位点位于复制末端附近。染色体二聚化导致在同一个分子中形成两个 的副本，导致快速的位点特异性重组并转换回染色体单体。迄今为止， 中缺乏染色体二聚体分辨率机制一直是一个谜。通过研究一种类似噬菌体的可移动遗传元件 LME-1，我们已经确定了一种以前未知的单重组酶 /Xer 系统，该系统不仅在整个 中广泛存在，而且其活性与两种重要的人类病原体的毒力有关。
