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SUMMARY The effect of irradiating donor cells, immediately before mating, upon the yield of recombinants has been investigated for male (F++Hfr) strains of Escherichia coli, which, owing to a mutational block (uvr), are unable to excise pyrimidine photoproducts. Despite the extreme sensitivity of the uvr- strains employed, as judged by colony formation, the yield of recombinants was surprisingly little affected by U.V. In particular, using an Hfr uvr; strain it was found that, after an initial fall to about 30-40% of that given by the unirradiated control, the yield of recombinants for both early and late markers declined with increasing dose at about the same rate as for the parent uvr+ strain. There was evidence of damage in the DNA transferred from irradiated males in that normal linkage of unselected markers was reduced, but the decline in linkage with increasing dose was the same for both uvr+ and uvr- strains. The yield of recombinants was nearly independent of the uvr phenotype of the F- parent. Thus although fertility and survival are closely correlated in the uvr+ Hfr, this correlation disappears in the uvr- male. Instead the U.V. sensitivity of the processes involved in chromosome transfer appears only slightly altered in these mutants despite the consider- able change in sensitivity as judged by colony-formation.

The Effects of Ultra-violet Irradiation on the Fertility of F+ and Hfr Strains of Escherichia coli K 12 Defective for the Repair of Damaged DNA