Gowrishankar Jayaraman
Laboratory of Bacterial Genetics, Centre for DNA Fingerprinting and Diagnostics, Hyderabad, India.
PLoS Genet. 2015 Jan 8;11(1):e1004909. doi: 10.1371/journal.pgen.1004909. eCollection 2015 Jan.
In bacterial cells, bidirectional replication of the circular chromosome is initiated from a single origin (oriC) and terminates in an antipodal terminus region such that movement of the pair of replication forks is largely codirectional with transcription. The terminus region is flanked by discrete Ter sequences that act as polar, or direction-dependent, arrest sites for fork progression. Alternative oriC-independent modes of replication initiation are possible, one of which is constitutive stable DNA replication (cSDR) from transcription-associated RNA-DNA hybrids or R-loops. Here, I discuss the distinctive attributes of fork progression and termination associated with different modes of bacterial replication initiation. Two hypothetical models are proposed: that head-on collisions between pairs of replication forks, which are a feature of replication termination in all kingdoms of life, provoke bilateral fork reversal reactions; and that cSDR is characterized by existence of distinct subpopulations in bacterial cultures and a widespread distribution of origins in the genome, each with a small firing potential. Since R-loops are known to exist in eukaryotic cells and to inflict genome damage in G1 phase, it is possible that cSDR-like events promote aberrant replication initiation even in eukaryotes.
在细菌细胞中，环状染色体的双向复制从单一的复制起点（oriC）开始，并在相对的终点区域终止，使得这对复制叉的移动在很大程度上与转录方向一致。终点区域两侧是离散的Ter序列，这些序列作为极性的，即方向依赖性的，复制叉前进的停滞位点。存在不依赖oriC的复制起始替代模式，其中之一是来自转录相关的RNA-DNA杂交体或R环的组成型稳定DNA复制（cSDR）。在这里，我讨论了与细菌复制起始的不同模式相关的复制叉前进和终止的独特特征。提出了两个假设模型：在所有生命王国中，复制叉对之间的正面碰撞是复制终止的一个特征，会引发双侧复制叉反转反应；并且cSDR的特征是细菌培养物中存在不同的亚群，以及基因组中起始点的广泛分布，每个起始点的启动潜力都很小。由于已知R环存在于真核细胞中并在G1期造成基因组损伤，因此即使在真核生物中，类似cSDR的事件也可能促进异常的复制起始。
