Bacteriophage T4 rnh (RNase H) Null Mutations: Effects on Spontaneous Mutation and Epistatic Interaction with rII Mutations

Abstract

ABSTRACT The bacteriophage T4 rnh gene encodes T4 RNase H, a relative of a family of flap endonucleases. T4 rnh null mutations reduce burst sizes, increase sensitivity to DNA damage, and increase the frequency of acriflavin resistance (Ac r ) mutations. Because mutations in the related Saccharomyces cerevisiae RAD27 gene display a remarkable duplication mutator phenotype, we further explored the impact of rnh mutations upon the mutation process. We observed that most Ac r mutants in an rnh + strain contain ac mutations, whereas only roughly half of the Ac r mutants detected in an rnh Δ strain bear ac mutations. In contrast to the mutational specificity displayed by most mutators, the DNA alterations of ac mutations arising in rnh Δ and rnh + backgrounds are indistinguishable. Thus, the increase in Ac r mutants in an rnh Δ background is probably not due to a mutator effect. This conclusion is supported by the lack of increase in the frequency of rI mutations in an rnh Δ background. In a screen that detects mutations at both the rI locus and the much larger rII locus, the r frequency was severalfold lower in an rnh Δ background. This decrease was due to the phenotype of rnh rII double mutants, which display an r + plaque morphology but retain the characteristic inability of rII mutants to grow on λ lysogens. Finally, we summarize those aspects of T4 forward-mutation systems which are relevant to optimal choices for investigating quantitative and qualitative aspects of the mutation process.