EFFECTS OF VACUUM UV AND UVC RADIATION ON DRY ESCHERICHIA-COLI PLASMID PUC19 .2. MUTATIONAL SPECIFICITY AT THE LACZ GENE

The mutational spectra at the lacZ gene, induced either by vacuum UV a t 160 nm or UVC at 254 nm in vacuum-dried preparations of Escherichia coli plasmid pUC19 DNA, have been characterized from 72 E. coli-propag ated mutants by DNA sequencing. In plasmids irradiated in vacuum, vacu um UV is five times more mutagenic than UVC. In the UV-induced mutants , base substitutions largely predominate, with GC --> AT (G, guanine; C, cytosine; A, adenine; T, thymine) transitions being the most abunda nt type of base change for vacuum UV (61%) and UVC (47%). Most of the GC --> AT transitions appear to occur at dipyrimidine sites, which are located at the non-transcribed DNA strand. Some, but not all, hot spo ts for GC --> AT transitions are identical for vacuum UV and UVC. Fram eshifts, resulting from a loss of the thymine residue, are specific fo r UVC (22%), and were not detected after treatment with vacuum UV. The y occur predominantly at thymine runs of the transcribed DNA strand. O nly a few deletions were detected following irradiation with vacuum UV (7.5%) and UVC (2%); however, their frequency is not enhanced compare d with the spontaneous mutation spectrum. The data confirm the importa nt role of base substitution mutations in UV-induced mutagenesis, whic h is not only valid for the UVC range, but extends towards the vacuum UV range.