ROLES OF THE HELICASE AND PRIMASE DOMAIN OF THE GENE-4 PROTEIN OF BACTERIOPHAGE-T7 IN ACCESSING THE PRIMASE RECOGNITION SITE

The 63 kDa gene 4 protein of bacteriophage T7 provides both helicase a nd primase activities, The C-terminal helicase domain of the gene 4 pr otein is responsible for DNA-dependent NTP hydrolysis and for hexamer formation, whereas the IV-terminal primase domain contains the zinc mo tif that is, in part, responsible for template-directed oligoribonucle otide synthesis. In the presence of beta,gamma-methylene dTTP, the pro tein forms a hexamer that surrounds and binds tightly to single-strand ed DNA and consequently is unable to translocate to primase recognitio n sites, 5'-GTC-3', or to dissociate from the molecule to which it is bound, Nonetheless, in the presence of beta,gamma-methylene dTTP, it c atalyzes the synthesis of pppAC dimers at primase sites on M13 DNA, Wh en bound to single-stranded DNA in the presence of beta,gamma-methylen e dTTP, the primase call function at recognition sites on the same mol ecule to which it is bound provided that a sufficient distance exists between the recognition site and the site to which it is bound. Furthe rmore, the primase bound to one DNA strand can function at a primase s ite located on a second DNA strand, The results indicate that the prim ase domain resides on the outside of the hexameric ring, a location th at enables It to access sites distal to its site of binding.