Abstract
An Escherichia coli strain with a mutation in the optA gene restricts the growth of bacteriophage T4 strains partially defective in gene 43 (DNA polymerase) or missing gene dda (DNA-dependent ATPase). The mutations in the dda gene inactivate a DNA-dependent ATPase that has been shown to have DNA helicase activity in vitro. The restriction of phage growth after infection of the optA bacterium is the result of a block in DNA replication. Evidently, the block arises from a defect in DNA unwinding.