Isolation of a Mutant Bacteriophage T7 Deleted in Nonessential Genetic Elements, Gene19.5andm

Abstract

Half of the 55 potential genes of bacteriophage T7 appear to be dispensable. One of the major obstacles in the study of these nonessential genes is the difficulty in obtaining mutants. During a study of genes involved in the packaging of bacteriophage T7, we hypothesized that some nonessential genes may be required for optimal growth. Mutant phages lacking such nonessential genes may form plaques but grow slowly. One gene located at the extreme right end of the linear T7 genome, gene19.5with no known mutants, and a genetic elementmresponsible for a unique hairpin end, were studied. Mutant T7 phages deleted in gene19.5andm(T7Δ19.5-M) were generatedin vivoby homologous recombination with a recombinant plasmid. This phage produces small plaques and the production of progeny phage particles per infected cell was reduced fourfold. Investigation of the intracellular DNA after infection with T7Δ19.5-Mshowed the persistence ofEscherichia coliDNA as well as delayed conversion of concatemers to unit-length T7 DNA. The inefficiency of concatemer processing confirmed the proposed function of the M-hairpin in duplication of the concatemer junction. Since it is not likely that the M-hairpin influences the degradation of host DNA, we propose that the gene19.5product is partly responsible for the degradation ofE. colichromosomal DNA.