THE INFLUENCE OF DNA-REPAIR BY OGT ALKYLTRANSFERASE ON THE DISTRIBUTION OF ALKYLNITROSOUREA-INDUCED MUTATIONS IN ESCHERICHIA-COLI

To determine the influence of DNA repair by Ogt alkyltransferase on th e distribution of alkylnitrosourea-induced mutations, we have analysed in Ogt-proficient and Ogt-deficient bacterial strains the DNA sequenc e changes of a total of 357 independent mutations occurring within the initial part of the loci gene of Escherichia coli. The majority (>80% ) of mutations induced by either N-ethyl-N-nitrosourea (ENU) or N-meth yl-N-nitrosourea (MNU) in the two genetic backgrounds were G:C --> A:T transitions, consistent with the predominant role of the O-6-alkylgua nine miscoding lesion in mutagenesis by alkylating agents. The analysi s of the distribution of G:C --> A:T transitions induced by ENU in Ogt (+) and Ogt(-) bacteria reveals an influence of the 5'-flanking base a t the level of repair by Ogt alkyltransferase. The Ogt protein appears more efficient at repairing O-6-ethylguanine lesions, which are flank ed 5' by a G or C, in agreement with previously reported data from our group for ethylmethane sulfonate. In contrast, no preference could be inferred for the repair of O-6-methylguanine lesions by Ogt protein. These results seem to indicate that the preference of the Ogt alkyltra nsferase to repair certain DNA sequences might be a Function of the si ze of the alkyl group. The importance of the alkyl group length has be en described also at the level of the (A)BC excinuclease machinery tha t seems to have a DNA sequence specificity opposite to that of Ogt alk yltransferase. (C) 1997 Wiley-Liss, Inc.