EFFECTS OF NITROUS-ACID TREATMENT ON THE SURVIVAL AND MUTAGENESIS OF ESCHERICHIA-COLI-CELLS LACKING BASE EXCISION-REPAIR (HYPOXANTHINE-DNA GLYCOSYLASE-ALK-A PROTEIN) AND OR NUCLEOTIDE EXCISION-REPAIR/

Deoxyinosine occurs in DNA by spontaneous deamination of adenine or by incorporation of dITP during replication, Hypoxanthine residues (HX) are mutagenic and give rise to A-T-->G-C transition, They are substrat es for the Escherichia coli product of the alkA gene, the 3-methyl-ade nine-DNA glycosylase II (ALK A protein), In mammalian cells and in yea st, HX is excised by the counterpart of ALK A protein, the ANPG or the MAG proteins respectively, We have investigated in vivo the contribut ion of the alkA gene to counteract the lethal and/or mutagenic effects of HX residues induced by nitrous acid treatment, Using an E.coli str ain allowing the detection of A-T-->G-C transition, we show that the a lkA mutant has a slightly increased spontaneous rate of mutation and a bout the same sensitivity when treated with HNO2 as compared with the wild-type strain, Using the E.coli alkA mutant carrying a multicopy pl asmid expressing the ALK A protein or the ANPG protein, we barely obse rve any effect of HNO2 treatment on sensitivity and mutation rate of t he bacteria, In contrast, the same experiment performed with a uvrA(-) strain, deficient in nucleotide excision repair (NER), shows that thi s mutant is extremely sensitive to HNO2 treatment, Futhermore, the sen sitivity and the spontaneous mutation rate observed in the double muta nt alkA(-) uvrA(-) are almost identical to those of the uvrA- mutant, Hence, NER has the major role in vivo for the repair of lethal and mut agenic lesions induced by HNO2.