ARE THERE DNA-DAMAGE CHECKPOINTS IN ESCHERICHIA-COLI

The concept of regulatory 'checkpoints' in the eukaryotic cycle has proved to be a fruitful one, Here, its applicability to the bacterial cell cycle is examined. A primitive DNA damage checkpoint operates in E. coli such that, after exposure to ultraviolet light, while excision repair occurs, chromosome replication continues very slowly with the production of discontinuous daughter strands. The slower the rate of excision of photoproducts, the greater the delay before the normal rate of DNA replication is restored, the additional time for repair ensuring that normal survival is maintained. A model is proposed in which replication rate is controlled by the ratio of RecA-coated to uncoated single stranded regions of DNA in the replication fork, There are also two cell division inhibitors SulA (=SfiA) and SfiC under the control of the SOS system and sensitive to DNA damage, but they are irrelevant to the survival of wild-type bacteria under normal conditions. In strains where SulA and SfiC do not operate, inhibition is not influenced by the rate of excision repair and so fails one of the criteria for a DNA damage checkpoint, namely the monitoring of the DNA for the level of residual damage.