No abstract

No abstract

The intracellular replicating form of T7 DNA is a concatemer in which linear genomes are joined head to tail by sharing 160-bp terminally repeated sequences. A unique hairpin (M-hairpin) end generated on the left side of the TR was proposed to be responsible for the duplication of the concatemer junction for efficient packaging. We characterized the defects caused by loss of the M-hairpin by constructing a recombinant T7 (T7Deltam) deleted in the m region. Initially, the intracellular growth rate of progeny phage was normal in T7Deltam infection. However, the titer of progeny phage was eventually reduced by two- to threefold and lysis was significantly delayed. The restriction fragment, LEDelta160, generated simultaneously with the double-strand cleavage at the onset of packaging reaction was found more or less at the same intensity in both T7(+) and T7Deltam infection at the beginning but preferentially in T7Deltam infection during the later phase of infection. These observations suggest that the DNA packaging of T7 proceeds on the intact concatemer junctions during the early stage of infection while the duplication of the concatemer junction by the M-hairpin seemed to be important during the later phase, presumably due to reduced replication. While the generation of the M-hairpin involves DNA replication, the loss of m did not reduce DNA synthesis, suggesting that the role of the M-hairpin as an origin of replication is minimal.

No 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, 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.