A 7-kDa region of the bacteriophage T7 gene 4 protein is required for primase but not for helicase activity.

Bacteriophage T7 gene 4 protein, purified from phage-infected cells, consists of a mixture of 56- and 63-kDa species that provides helicase and primase activities required for T7 DNA replication. The 56-kDa species has been purified independently of the colinear 63-kDa species. Like a mixture of the two proteins, the 56-kDa protein binds single-stranded DNA in the presence of dTTP, catalyzes DNA-dependent hydrolysis of dTTP, and has helicase activity. In contrast to the mixture, the 56-kDa protein cannot catalyze template-dependent RNA primer synthesis. In the absence of a DNA template, both the 56-kDa protein and the mixture of the two species synthesize low levels of diribonucleotide. A putative "zinc finger" present near the amino terminus of the 63-kDa protein but absent from the 56-kDa protein may play a major role in the recognition of primase sites in the template.