33 Mutation Research, 42 (1977) 33-44 © Elsevier/North-Holland Biomedical Press MUTAGENIC DNA REPAIR IN ESCHERICHIA COLI. V. MUTATION FREQUENCY DECLINE AND ERROR·FREE POST-REPLICATION REPAIR IN AN EXCISION-PROFICIENT STRAIN M.H.L. GREEN, B.A. BRIDGES, J.E. EYFJORD and W.J. MURIEL M.R.C. Cell Mutation Unit, University of Sussex, Folmer, Brighton BN 1 9QG (Breat Britain) (Received April 12th, 1976) (Revision received July 15th, 1976) (Accepted September 9th, 1976) Summary Mutation frequency decline (MFD) is an irreversible loss of newly-induced suppressor mutations occurring in excision-proficient Escherichia coli during a short period of incubation in minimal medium before plating on broth- or Casa- mino acids-enriched selective agar. It is known that MFD of UV-induced muta- tions may occur before DNA containing pre-mutagenic lesions is replicated, but we conclude that MFD can also occur after the damaged DNA has been repli- cated on the basis of the following evidence. (1) Mutation fixation in rich me- dium (i.e., loss of susceptibility to mutation frequency decline) with ethyl methanesulphonate mutagenesis begins immediately, whereas with UV it is de- layed for 20-30 min. (2) The delay in mutation fixation after UV can be ex- plained neither by inhibition of DNA replication nor by a delay in the appear- ance of error-prone repair activity in the irradiated population. (3) MFD at later times after UV irradiation is more rapid and is less strongly inhibited by caffeine than is MFD immediately after irradiation. (4) Excision is virtually complete 20 min after 3 J m? UV but at that time virtually all mutations are still susceptible to MFD. We have presented evidence elsewhere that in bacteria there is an alternative error-free excision-dependent type of post-replication repair of potentially mutagenic daughter strand gaps. We suggest that this process is inhibited at tRN A loci in the presence of nutrient broth or Casamino acids, possibly be- cause of a broth-dependent change in the structure of the single-stranded region including the tRNA locus. Introduction When Escherichia coli WP2 is exposed to ultraviolet light (UV) and assayed for the induction of Trp+ mutants the yield is 5 to 10 times greater if the plates