Isolation of a mutant bacteriophage T7 deleted in nonessential genetic elements, gene 19.5 and m

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, gene 19.5 with no known mutants, and a genetic element m responsible for a unique hairpin end, were studied. Mutant T7 phages deleted in gene 19.5 and m (T7 delta 19.5-M) were generated in vivo by 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 T7 delta 19.5-M showed the persistence of Escherichia coli DNA 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 gene 19.5 product is partly responsible for the degradation of E. coli chromosomal DNA.