DNA HELICASE REQUIREMENTS FOR DNA-REPLICATION DURING BACTERIOPHAGE-T4INFECTION

The lytic bacteriophage T4 uses multiple mechanisms to initiate the re plication of its DNA. Initiation occurs predominantly at replication o rigins at early times of infection, but there is a switch to genetic r ecombination-dependent initiation at late times of infection. The T4 i nsertion-substitution system was used to create a deletion in the T4 d da gene, which encodes a 5'-3' DNA helicase that stimulates both DNA r eplication and recombination reactions in vitro. The deletion caused a delay in T4 DNA synthesis at early times of infection, suggesting tha t the Dda protein is involved in the initiation of origin-dependent DN A synthesis. However, DNA synthesis eventually reached nearly wild-typ e levels, and the final number of phages produced per bacterium was si milar to that of the wild type. When the dda mutant phage also contain ed a mutation in T4 gene 59 (a gene normally required only for recombi nation-dependent DNA replication), essentially no DNA was synthesized. Recent in vitro studies have shown that the gene 59 protein loads a c omponent of the primosome, the T4 gene 41 DNA helicase, onto DNA. A mo lecular model for replication initiation is presented that is based on our genetic data.