Sequence analysis of ultraviolet-induced mutations in M13lacZ hybrid phage DNA

We have studied the specificity of ultraviolet (u.v.) mutagenesis in single-stranded DNA phage by analyzing u.v.-induced forward mutations in the lac insert of M13mp2 hybrid phage. Sequence analysis of 114 lac mutants derived from u.v.-irradiated phage grown in u.v.-irradiated cells showed that ultraviolet induces mainly single-nucleotide substitutions and deletions in progeny phage DNA. A total of 74% of the single-base substitution mutations occurred at sites of adjacent pyrimidines in the single-stranded DNA, with both T → C and C → T transitions predominating in the u.v. spectrum. Single-nucleotide deletion mutations occurred preferentially in tracts of repeated pyrimidine nucleotides. Tandem, double-base substitutions did not represent a major class of u.v.-induced mutations, but nearly 10% of mutant clones contained multiple, non-tandem nucleotide changes.