Sequence analysis of lacZ(-) mutations induced by ion beam irradiation in double-stranded M13mp18DNA.

While M13mp18 double-stranded DNA was irradiated with ion beam, and transfected intoE. coli JM103, a decrease of transfecting activity was discovered. The lacZ(-) mutation frequency at 20% survival could reach (3.6-16.8) x 10(4), about 2, 3-10 times that of unirradiated M13DNA. Altogether, 27 IacZ(-)mutants were selected, 10 of which were used for sequencing. 7 of the sequenced mutants show base changes in 250-bp region examined (the remaining 3 mutants probably have base changes outside the regions sequenced). 5 of the base-changed mutants contain more than one mutational base sites (some of them even have 5-6 mutational base sites in 250-bp region examined); this dense distribution of base changes in polysites has seldom been seen in X-rays, Y-rays or UV induced DNA mutations. Our experiments also showed that the types of base changes include transitions(50%), transversions (45%) and deletion (5%); no addition or duplication was observed. The transitions were mainly C-->T and A-->G; the transversions were mainly C-->A and C-->G. The mutations involving cytosine residue (in the template strand) constitute about 60% of all the base changes observed. In comparison with the surrounding sequences of mutational base sites, the base located between TG and CT is found to be easily substituted.