Oxidative damage and induced mutations in M13mp2 phage DNA exposed to N-nitrosopyrrolidine with UVA radiation

Abstract

N-Nitrosopyrrolidine (NPYR) is carcinogenic in rodents and undergoes α-hydroxylation upon microsomal CYP450 metabolism, giving rise to mutations. Previously, we reported the direct mutagenicity of NPYR, under ultraviolet A (UVA) irradiation, towards Salmonella typhimurium and phage M13mp2. In the present study, we measured the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) in a replicative form of M13mp2 DNA exposed to NPYR plus UVA. Formation of 5-hydroxy-2'-deoxycytidine in calf thymus DNA treated with NPYR plus UVA was also observed. Singlet oxygen is likely to account for the formation of 8-oxodGuo. We analyzed the spectrum of mutations in lacZα of M13mp2 phages produced on transfecting Escherichia coli with the replicative form of phage DNA that had been treated with NPYR plus UVA. The role of oxidative DNA damage in mutagenesis was explored using mutM-proficient and -deficient E.coli strains as the hosts. A higher level of mutation was observed with the mutM-deficient host than with the -proficient host. Base substitutions at GC pairs predominated in both mutM-proficient and -deficient hosts. With the mutM-deficient host, we observed an overall increase in the percentage of GC→TA transversions. In addition we noted that there were fewer GC→TA transitions that in the mutM-proficient host. With these hosts, different hot spots were observed and a new GC→TA hot spot was produced. The formation of 8-oxodGuo in DNA, which is known to induce GC→TA transversion, may contribute to mutagenesis by NPYR plus UVA.