Mutagenesis and mutation transfer induced by ultraviolet light in plasmid-cloned DNA

We describe here simple techniques for increasing the frequency of UV-induced mutations in a DNA fragment cloned in plasmid pBR322. Irradiation of both the host and the plasmid DNA before transformation is necessary to produce new mutations in the plasmid DNA, presumably because the UV-damaged pBR322 replicon cannot efficiently induce the error-prone repair pathway of Escherichia coli. In contrast, UV irradiation of the plasmid DNA alone before transformation primarily causes the transfer of preexisting mutations from the host chromosome to homologous DNA present in the plasmid. The only other kind of mutants obtained were large deletions of the plasmid DNA. Two chromosomal mutations from the host galK gene and one from the lacZ gene have been transferred to the plasmid by UV irradiation of the plasmid DNA alone. The technique can thus be of general use.