Enhanced resistance to nitrosoguanidine killing and mutagenesis in a DNA gyrase mutant of Escherichia coli.

The role of DNA gyrase in handling DNA damages induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was examined with two Escherichia coli strains, KL161 and KL166. The two strains are isogenic except that KL166 harbors a mutation at the nalA (gyrA) locus which specifies one of the two subunits of DNA gyrase. We treated the two strains with several different types of mutagenic agents and found the nalA strain to be highly resistant to MNNG-induced killing and mutagenic effects as compared with the parental strain. The MNNG resistance was specific, since the two strains were about equally sensitive to methyl methane sulfonate, ethyl methane sulfonate, and UV and gamma radiations. We pulse-labeled the two strains with [(3)H]uridine and (14)C-amino acids after MNNG treatment to analyze RNA and protein synthetic rates. The pulse-labeled proteins were also separated on polyacrylamide gels. The results show that pulse-labeled RNA and proteins persisted in the nalA strain but declined rapidly in the parental strain after MNNG treatment. We compared membrane-free nucleoid preparations from the two strains by sucrose density gradient centrifugation and found a difference in nucleoid organization between the two strains. The nucleoid of the nalA strain, unlike that of the parental strain, may have a highly ordered structure, as indicated by its resistance to ethidium bromide-induced relaxation. The ability of the two strains to express an adaptive response to MNNG was determined. We found that the resistance to MNNG killing and mutagenesis by the nalA strain cannot be further increased by adaptive treatment. These results suggest that an alteration in DNA gyrase may have profound effects on E. coli chromosome organization and base methylation by MNNG.