Marker rescue from UV-irradiated bacteriophages: A UV-sensitive region in bacteriophage T2

Abstract

Marker rescue was studied with T2 and T4 strains as irradiated donor strains and T2 am39 rIIA or T2 am47 gt29 am55 as helper strains. After selection of one marker from the donor strain, the frequency of the second donor marker was determined (corescue frequency). Corescue frequencies decreased with increasing doses of UV irradiation. In the region am47- am55 there was no difference between frequencies of corescue from T2 and T4 as donor strains. In the region am39- rIIA, frequencies of corescue from T2 were much lower than from T4 and could be ascribed to rescue of the two markers by independent rescue events. To explain the difference between T2 and T4, the following possibilities were investigated: (1) protection of a UV-sensitive site in T4 by a T4 gene product; (2) influence of partial exclusion of helper strain T2 by T4, and (3) a difference between T2 and T4 with respect to UV sensitivity in the region am39- rIIA. The first possibility could be rejected. Exclusion might play a minor role in corescue from T4. Probably, the region am39- rIIA in T2 is more susceptible to UV damage than in T4, presumably owing to a higher content of potential thymine dimers. The mean lengths of rescued DNA fragments at various doses of UV irradiation were estimated by using the observed corescue frequencies. Extrapolation of the relation between UV dose and length of rescued DNA fragments predicts a UV dose of 360–530 PLH required for rescue of DNA fragments of about two map units in the region of T4 gene α gt.