Mutational specificity of gamma-rays differs for the same target in plasmid DNA and double-stranded (RF) M13 bacteriophage DNA.

In the lacZ alpha gene of a pUC plasmid a 144 bp insert was cloned as target for mutagenesis. Irradiation of the plasmid in a diluted aqueous solution by 60Co gamma-rays under oxic conditions leads to a very specific mutation spectrum. The predominant type of mutation was a C/G to A/T transversion (29 out of 47 mutants) whereas C/G to G/C transversions were found 7 times and C/G to T/A transitions 10 times. Only one frameshift could be observed which was a deletion of an A/T base pair. The mutations were not randomly distributed along the mutation target but show a strong preference for a certain DNA sequence in which two thirds of the mutations were scored. In this DNA area a hotspot (24 of the 47 mutants) for mutagenesis was located and within 6 bp next to this hotspot another seven mutations were scored. The mutation spectrum in the same mutation target as part of double-stranded (RF) M13 phage was published before. In both systems the mutational hotspot is located at the same site, but the predominant type of mutation is different. In the M13 system the C/G to G/C transversion was the most important event.