THE ROLE OF MUTATION FREQUENCY DECLINE AND SOS REPAIR SYSTEMS IN METHYL METHANESULFONATE MUTAGENESIS

Methyl methanesulfonate (MMS) is an S(N)2 type allkylating agent which predominantly methylates nitrogen atoms in purines, Among the methyla ted bases 3meA and 3meG are highly mutagenic and toxic. The excision o f these lesions leads to the formation of apurinic (AP) sites and subs equently to AT-->TA or GC-->TA transversions. The in vivo method based on phenotypic analysis of Arg(+) revertants of Escherichia coil K12 a nd sensitivity to T4 nonsense mutants has been used to estimate the sp ecificity of MMS induced mutations. In the E. coli arg(-)his(-)thr(-) (AB1157) strain MMS induces argE3(oc) --> Arg(+) revertants of which 7 0-80% arise by supL suppressor formation as a result of AT-->TA transv ersions. The remaining 20-30% arise by supB and supE(oc) suppressor fo rmation as a result of GC-->AT transitions. The level of AT-->TA trans versions 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 repai r was discovered in UV induced mutations. We describe the mutation fre quency decline phenomenon for MMS mutagenesis. MMS is a very efficient inducer of the SOS response and a umuDC dependent mutagen. In MMS tre ated E. coli cells mutated in umuDC genes the class of AT-->TA transve rsions dramatically diminishes. A plasmid bearing UmuD(D')C proteins c an supplement chromosomal deletion of umuDC operon: a plasmid harbouri ng umuD'C is more efficient in comparison to that harbouring umuDc. Mo reover, 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' prote in.