Reversions of the two missense and one nonsense trp-mutations induced by UV-light or methyl methanesulfonate in E. coli wild type and its polAi and uvrE502 mutants.

Two missense mutations, trpA58 and trpA78, and one nonsense mutation-trp-ochre, were used to determine the types of base-pair substitution caused by ultra, violet irradiation and methyl methanesulfonate (MMS) in Escherichia coli. UV irradiation of the wild-type bacteria led to the formation of revertants mainly arising as a result of GC yields AT transitions (suppressor revertants of the trpA58 mutant). True revertants of the trp- mutant (arising via transitions of AT pairs) and 5-methyl tryptophan-sensitive (MT-s) Trp+ of the trpA78 mutant (arising via unidentified transversions) occurred at a lower frequency. The polAI mutation did not change the frequency of the UV-induced transitions GC yields AT or that of the substitutions of the AT pairs. The uvrE502 mutation significantly increased the frequency of the UV-induced revertants arising via the transition GC yields AT. Treatment of the wild-type bacteria with MMS resulted in the formation of revertants mainly due to the GC yields AT substitution, and with a lower frequency to the AT yields GC transitions. MMS also induced, with a low frequency, some transversions. The frequency of the MMS-induced GC yields AT transitions was enhanced in the uvrE502 mutant. On the other hand, the uvrE502 mutation eliminated or significantly lowered MMS-induced revertants arising as a result of AT yields GC transitions or transversions.