分区系统确保细菌低拷贝数复制子的稳定遗传,比如染色体,铬,和巨型质粒。这些基因座由编码分区蛋白A和B的两个基因组成,和至少一个parS着丝粒样序列。在铬和巨质粒中,分区系统通常位于复制系统附近。这种共定位的一个极端例子是α变形菌repABC复制子,其中分区(repAB)和复制(repC)基因形成单个操纵子,parS序列通常位于这些基因附近。在这项研究中,我们表征了在嗜氨基副球菌(Rhodobacterales)大质粒pAMI4(438kb)中发现的更复杂的repABC系统。除了具有单个parS位点的repABC操纵子之外,该复制子具有位于repC下游11.5kb的2kb非编码基因座,它包含三个额外的parS重复(3parS)。我们证明了3parS在体外被分区蛋白B结合,并且对于体内适当的pAMI4分配至关重要。在寻找相似的基因座时,我们对其他repABC复制子的parS分布进行了比较分析。这揭示了红杆菌属和根瘤菌属中parS定位的不同模式。然而,在这两个分类顺序中,parS位点几乎总是位于repABC操纵子内部或附近。在pAMI4的近亲中未发现其他3parS样基因座。这种基因座的另一个不依赖于进化的实例被鉴定为异型根瘤菌和相关复制子的2号染色体中的保守特征。重要性repABC复制/分配基因座广泛存在于α变形杆菌的染色体外复制子中。它们在进化上是多样的,受多层自我调节,并负责维护不同类型的复制子,如质粒(例如,农杆菌pTi和pRi致瘤和发根质粒),巨质粒(例如,中华根瘤菌pSymA和pSymB)和必需铬(例如,农杆菌的次级染色体,布鲁氏菌和红细菌)。在这项研究中,我们对repABC系统的非典型分区相关成分进行了功能分析,3parS位点,在嗜胺性疟原虫大质粒pAMI4中发现。我们还确定了不同组repABC复制子中parS着丝粒样位点的分布模式,并发现了其他不相关的3parS样位点,被忽视了。我们的发现提出了有关差异parS分布的生物学原因的问题,这可能反映了repABC操纵子调节的变化以及属于repABC家族的复制子的不同复制和分区模式。
Partitioning systems ensure the stable inheritance of bacterial low-copy-number replicons, such as chromosomes, chromids, and megaplasmids. These loci consist of two genes encoding partition proteins A and B, and at least one parS centromere-like sequence. In chromids and megaplasmids, partitioning systems are often located in the vicinity of replication systems. An extreme example of this co-localization are alphaproteobacterial
repABC replicons, where the partition (repAB) and replication (repC) genes form a single operon, with parS sequences usually positioned in close proximity to these genes. In this study, we characterized a more complex
repABC system found in Paracoccus aminophilus (Rhodobacterales) megaplasmid pAMI4 (438 kb). Besides the
repABC operon with a single parS site, this replicon has a 2-kb non-coding locus positioned 11.5 kb downstream of repC, which contains three additional parS repeats (3parS). We demonstrated that 3parS is bound by partition protein B in vitro and is essential for proper pAMI4 partitioning in vivo. In search of similar loci, we conducted a comparative analysis of parS distribution in other repABC replicons. This revealed different patterns of parS localization in Rhodobacterales and Rhizobiales. However, in both these taxonomic orders, parS sites are almost always located inside or close to the
repABC operon. No other 3parS-like loci were found in the closest relatives of pAMI4. Another evolutionarily-independent example of such a locus was identified as a conserved feature in chromosome 2 of Allorhizobium vitis and related replicons. IMPORTANCE The repABC replication/partitioning loci are widespread in extrachromosomal replicons of Alphaproteobacteria. They are evolutionarily diverse, subject to multi-layer self-regulation, and are responsible for the maintenance of different types of replicons, such as plasmids (e.g., Agrobacterium pTi and pRi tumorigenic and rhizogenic plasmids), megaplasmids (e.g., Sinorhizobium pSymA and pSymB) and essential chromids (e.g., secondary chromosomes of Agrobacterium, Brucella and Rhodobacter). In this study, we functionally analyzed an atypical partition-related component of
repABC systems, the 3parS locus, found in the P. aminophilus megaplasmid pAMI4. We also identified parS centromere-like site distribution patterns in different groups of repABC replicons and found other unrelated 3parS-like loci, which had been overlooked. Our findings raise questions concerning the biological reasons for differential parS distribution, which may reflect variations in repABC operon regulation as well as different replication and partition modes of replicons belonging to the repABC family.