Abstract
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.
Highlights
Many features of chromosomal DNA replication are shared across the three kingdoms of life, including initiation from discrete origins, bidirectional fork progression, and termination by merger of opposing replication forks [1]
Whereas replication in eukaryotes is initiated from multiple origins on linear chromosomes, in bacteria most often there is a single circular chromosome whose replication is initiated from an oriC locus and proceeds bidirectionally for forks to meet in an antipodal terminus region
This review explores the dynamics of fork progression and termination in Escherichia coli bacterial cells exhibiting oriC-dependent and oriC-independent replication initiation to support two new concepts: (i) that when pairs of forks collide, bilateral fork reversal reactions take place; and (ii) that constitutive stable DNA replication (cSDR) is characterized by stochastic replication initiation events distributed genome-wide
Summary
Laboratory of Bacterial Genetics, Centre for DNA Fingerprinting and Diagnostics, Hyderabad, India. Overview: 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. 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. 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 Rloops 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
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