C G TO A/T TRANSVERSIONS REPRESENT THE MAIN TYPE OF MUTATION INDUCED BY GAMMA-IRRADIATION IN DOUBLE-STRANDED M13MP10 DNA IN A NITROGEN-SATURATED SOLUTION/

To get more insight into the possible mutagenic consequences of DNA da mage induced by radiation-generated H radicals (.H), a nitrogen-satura ted solution of double-stranded (ds) M13mp10 DNA in phosphate buffer w as irradiated with gamma-rays. Under these conditions 55% of the DNA-d amaging species consists of H radicals and 45% of OH radicals (.OH). T he mutations were investigated in a 144-bp mutational target sequence inserted into the lacZalpha gene. A very specific mutation spectrum wa s obtained with respect to the type of mutations. Twenty out of the 28 radiation-induced mutations were C/G to A/T transversions; the remain ing 8 mutations were 4 C/G to G/C transversions, 2 C/G to T/A transiti ons, one T/A to A/T transversion and only one -1 bp deletion. The muta tions were rather randomly distributed along the 144-bp mutation targe t sequence with no clear mutational hot spots. When these results are compared with those we have obtained previously after irradiation of d s M13mp10 DNA under O2 (100% .OH) or N2O (90% .OH; 10% .H) (Hoebee et al., 1988, 1989), the data strongly suggest that H radicals may be res ponsible for the observed C/G to A/T transversions but not for -1 bp d eletions.