Abstract
DNA damage checkpoints exist to promote cell survival and the faithful inheritance of genetic information. It is thought that one function of such checkpoints is to ensure that cell division does not occur before DNA damage is repaired. However, in unicellular organisms, rapid cell multiplication confers a powerful selective advantage, leading to a dilemma. Is the activation of a DNA damage checkpoint compatible with rapid cell multiplication? By uncoupling the initiation of DNA replication from cell division, the Escherichia coli cell cycle offers a solution to this dilemma. Here, we show that a DNA double-strand break, which occurs once per replication cycle, induces the SOS response. This SOS induction is needed for cell survival due to a requirement for an elevated level of expression of the RecA protein. Cell division is delayed, leading to an increase in average cell length but with no detectable consequence on mutagenesis and little effect on growth rate and viability. The increase in cell length caused by chronic DNA double-strand break repair comprises three components: two types of increase in the unit cell size, one independent of SfiA and SlmA, the other dependent of the presence of SfiA and the absence of SlmA, and a filamentation component that is dependent on the presence of either SfiA or SlmA. These results imply that chronic checkpoint induction in E. coli is compatible with rapid cell multiplication. Therefore, under conditions of chronic low-level DNA damage, the SOS checkpoint operates seamlessly in a cell cycle where the initiation of DNA replication is uncoupled from cell division.
Highlights
Unrepaired DNA double-strand breaks (DSBs) are a lethal form of damage
This study demonstrates that a single, efficiently repaired, DSB per chromosome per replication cycle is sufficient to induce the SOS response of E. coli and that this induction is required for cell viability
It is possible that the need for an elevated level of RecA protein reflects the repetitive nature of the damage induced since a naıve cell encountering a DSB will not have an induced level of RecA protein and normally survives the damage
Summary
Unrepaired DNA double-strand breaks (DSBs) are a lethal form of damage. In Escherichia coli, DNA double-strand break repair (DSBR) is carried out by homologous recombination, a pathway that has been conserved in evolution from bacteria to humans. Recombination mediates repair of a damaged DNA molecule using an undamaged template, which is usually the sister chromosome generated during DNA replication [1] This reaction is centrally catalyzed by the RecA protein in E. coli and by its homologue Rad in eukaryotes [2]. In E. coli, the main DNA damage checkpoint is the SOS response [5] It is controlled by the same RecA protein that mediates homologous recombination. FtsZ polymerizes to form a ring-like structure at mid-cell where it acts as a scaffold for other division proteins Another system that inhibits cell division is nucleoid occlusion, which is mediated by the SlmA protein that prevents the polymerization of FtsZ filaments into productive FtsZ rings in the presence of DNA [15,16,17]. There is an increase in very long cells that occurs in the presence of either SfiA or SlmA and is only significantly reduced in the double mutant
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
