RECOVERY OF DNA-REPLICATION IN UV-DAMAGED ESCHERICHIA-COLI

Following exposure to UV light DNA replication stops and then resumes. The SOS response is required for the restoration of replication. Replication recovery occurs in lexA(Ind) cells carrying a high constitutive level of RecA protein. Replication is also affected by UmuCD proteins, photoreactivation, and excision repair. In addition, there is a constitutive and recA independent way to replicate over UV photoproducts associated with the production of gaps in daughter DNA strands. There are two ways to account for the replication in UV-irradiated cells. A stalled replication fork can be reactivated. Alternatively, a replication fork could be destroyed irreparably, with no available way to complete the round of replication. In that case, postirradiation replication could be due exclusively to replication forks assembled de novo at the origin(s). Changes in replication initiation are observed following UV irradiation. Initiations are first inhibited and then stimulated. They become independent of de novo protein synthesis and sometimes do not stop in dnaA(ts) mutants shifted to 42-degrees-C. Although the inducible functions are involved in the recovery of replication at different levels of UV damage, some modifications of the replicaton initiation mechanism appear to be specific to severely damaged cells. Such modifications seem to include the dnaA(ts) independence for initiations and the transient initiation inhibition. RecA protein can be directly involved both in the modification of initiation and in reactivation of the stalled replication forks. Although the restoration of replication depends on the SOS response a synthesis of some protein(s) that do not belong to the LexA regulon seems to be required as well. These proteins can be under RecA control and one of their functions may be to inhibit the rnhA gene. Certain recA mutations may selectively affect different mechanisms of the replication recovery (namely, recA430, recA727, recA718, recA1730). Overproduction of the photoreactivating enzyme in the dark could influence UmuCD activity in replication. The UmuCD function appears to be blocked in strains carrying the dnaE1026 mutation or overproducing the dnaQ protein. For some unknown reason the UmuCD-associated replication mechanism is the only one available for phage with damaged DNA.