Methyl methanesulfonate (MMS) is an SN2 type alkylating agent which predominantly methylates nitrogen atoms in purines. Among the methylated bases 3meA and 3meG are highly mutagenic and toxic. The excision of these lesions leads to the formation of apurinic (AP) sites and subsequently to AT-->TA or GC-->TA transversions. The in vivo method based on phenotypic analysis of Arg+ revertants of Escherichia coli K12 and sensitivity to T4 nonsense mutants has been used to estimate the specificity of MMS induced mutations. In the E. coli arg-his-thr- (AB1157) strain MMS induces argE3(oc)-->Arg+ revertants of which 70-80% arise by supL suppressor formation as a result of AT-->TA transversions. The remaining 20-30% arise by supB and supE(oc) suppressor formation as a result of GC-->AT transitions. The level of AT-->TA transversions decreases during starvation. This is a consequence of action of the repair mechanism called mutation frequency decline. This system which is a transcription coupled variant of nucleotide excision repair was discovered in UV induced mutations. We describe the mutation frequency decline phenomenon for MMS mutagenesis. MMS is a very efficient inducer of the SOS response and a umuDC dependent mutagen. In MMS treated E. coli cells mutated in umuDC genes the class of AT-->TA transversions dramatically diminishes. A plasmid bearing UmuD(D')C proteins can supplement chromosomal deletion of umuDC operon: a plasmid harbouring umuD'C is more efficient in comparison to that harbouring umuDC. Moreover, plasmids isolated from MMS treated and transiently starved E. coli AB1157 cells harbouring umuD(D')C genes have shown the repair of AP sites by a system which involves the UmuD'C or at least UmuD' protein.