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 damage induced by radiation-generated H radicals (.H), a nitrogen-saturated solution of double-stranded (ds) M13mp10 DNA in phosphate buffer was irradiated with gamma-rays. Under these conditions 55% of the DNA-damaging species consists of H radicals and 45% of OH radicals (.OH). The mutations were investigated in a 144-bp mutational target sequence inserted into the lacZ alpha gene. A very specific mutation spectrum was obtained with respect to the type of mutations. Twenty out of the 28 radiation-induced mutations were C/G to A/T transversions; the remaining 8 mutations were 4 C/G to G/C transversions, 2 C/G to T/A transitions, one T/A to A/T transversion and only one -1 bp deletion. The mutations were rather randomly distributed along the 144-bp mutation target sequence with no clear mutational hot spots. When these results are compared with those we have obtained previously after irradiation of ds 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 responsible for the observed C/G to A/T transversions but not for -1 bp deletions.