Mutation frequency decline in a rel strain of E. coli coli B/r

Mutation frequency decline (MFD) occurs among certain suppressor mutations induced by UV irradiation of E. coli. When irradiated bacteria are incubated in conditions less than optimal for protein synthesis prior to plating, a large majority of the potential mutants are rapidly lost. For several years the belief has existed that MFD results because stringent control of transcription at tRNA genes may influence excision repair of premutational photoproducts at these sites. We have constructed a rel mutant of E. coli B/r strain WU3610, verified that it has a typical phenotype of a relaxed strain in terms of RNA and ppGpp synthesis, and measured MFD. The kinetics of MFD were the same in WU3610 and WU3610 rel. Thus stringent control is not essential to MFD and there is no reason to believe that moderation of transcription at tRNA genes is integral to the MFD mechanism.