Defects in concatemer processing of bacteriophage T7 DNA deleted in the M-hairpin region

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.