THE MUTATIONAL SPECIFICITY OF 1-NITROSO-6-NITROPYRENE IN THE LACI GENE OF ESCHERICHIA-COLI STRAINS DEFICIENT IN NUCLEOTIDE EXCISION-REPAIR

We have examined the mutational specificity of 1-nitroso-6-nitropyrene (1,6-NONP), an activated metabolite of the carcinogen 1,6-dinitropyre ne, in the lacI gene of Escherichia coli strains which are deficient i n nucleotide excision repair (strain NR6113, Delta uvrB; strain CM6114 , Delta uvrB, plasmid pKM101), Separate collections of lad-mutations a nd dominant lacI(-d) mutations, which contain DNA sequence alterations in the region of the lad gene that encodes the DNA binding domain of the lad repressor, were made following 1,6-NONP treatment, The DNA seq uence of 418 mutations was determined, of which 228 were lacI(-) mutat ions and 190 were lacI(-d) mutations. Ninety three percent of the indu ced point mutations occurred at G:C residues, -(G:C) frameshifts were the dominant mutational class in the lacI(-) collections of both NR611 3 and CM6114, and in the lacI(-d) collection of NR6113, The frameshift mutations occurred preferentially in runs of guanine residues and the ir frequency increased markedly with the length of the reiterated sequ ence, In strain CM6114, which contained the plasmid pKM101, there was a marked stimulation in the frequency of G:C-->T:A transversions that was particularly apparent in the lacI(-d) collection. We discuss model s which might account for the apparent differences in mutational speci ficity resulting from the presence of the UmuD/C and MucA/B proteins, The results suggest that major classes of mutation are recovered in bo th the lacI(-) and lacI(-d) collections. However, the proportions of t he major classes of mutations within the two collections can differ si gnificantly, Depending on the genetic background of the host strain, t he relative ratios of base substitutions to frameshift mutations in th e lacI(-d) target can differ by almost an order of magnitude as compar ed with the lacI(-) target, This is primarily a function of the relati ve mutational target size of the different classes of mutation.