Biochemical and Biophysical Research Communications Ultraviolet Light-induced Recombination

Stimulation of recombination by ultraviolet (UV) irradiation was discovered by Jacob and Wollmanl using the phage a. Subsequently recombination in both prokaryotic and eukaryotic organisms has been shown to be stimulated by W light and other agents acting on DNA. A close relationship between the repair of damaged DNA and recombination was established directly by the isolation of ret mutants of Escherichia coli which are defective in both functions 2 . Other types of mutants deficient in DNA-repair have been isolated, but most of these exhibit normal recombination frequencies. The role of each of these "repair" genes in W-stimulated recombination has not been determined, In my work the effect of W light on recombination in mutants of E. coli -deficient in excision-repair of DNA has been examined. uvrA and uvrB mutants lack an endonuclease which makes a single-strand break in DNA containing a Winduced pyrimidine dimer 3 . uvrC mutants have this activity3 but are unable to make the second break which releases an oligonucleotide containing the dimer4. DNA polymerase I is believed to replace the DNA lost by excision; this enzyme is lacking in polA mutants5. DNA ligase is believed to complete excisionrepair by making the last phosphodiester linkage. The conversion of abortive (potential) to complete transductants provides a convenient and sensitive assay for W-stimulated recombination6. Usually less than 10% of the cells which receive a given gene from a transducing phage form stable transductants containing that gene. Following W-irradiation, the number of complete transductants increases. This is a consequence of increased recombination which converts abortive to stable transductants7. Certain mutants of E. a6, Salmonella typhimurium8, and Streptococcus pyogenes 9 defective in DNA-repair fail to exhibit thfs UV-stimulated conversion. The results of the experiments presented here show that in transduction,