Error-prone DNA repair in Escherichia coli. 4

CM 1031, a dnaB mutant of E. coli with normal resistance to UV- and gamma-radiation, has previously been shown to be deficient at permissive temperatures in host cell reactivation of phage lambda and to show induction of Weigle reactivation of lambda at abnormally low UV doses. The suggestion that this might be due to delayed excision of UV-photoproducts has been confirmed biochemically. In addition the dnaB strain at permissive temperatures shows a greatly enhanced induction of Thr+ mutants by both UV- and gamma-radiation. There is also a smaller effect on the spontaneous mutation rate. At restrictive temperature potentially mutagenic UV but not gamma-radiation damage in CM 1031 is repaired in an error-free manner leading to a decline in the number of induced mutants and an increase in survival. It is suggested that the slow excision phenotype of CM 1031 may be critical in preventing the operation of an excision-dependent error-free post-replication repair pathway specific for potentially mutagenic daughter strand gaps. (author)