Characterization of the helicase and primase activities of the 63-kDa component of the bacteriophage T7 gene 4 protein.


Leading and lagging strand DNA synthesis at the replication fork of ...
Leading and lagging strand DNA synthesis at the replication fork of bacteriophage T7 DNA requires the helicase and primase activities of the gene 4 protein. Gene 4 protein consists of two colinear polypeptides of 56- and 63-kDa molecular mass. We have demonstrated previously that the 56-kDa protein possesses helicase but lacks primase activity (Bernstein, J. A., and Richardson, C. C. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 396-400). The 63-kDa gene 4 protein has now been purified from extracts of T7-infected cells. The preparation contains 5-10% contaminating 56-kDa protein, as shown by Western analysis using polyclonal antibodies to the purified 56-kDa protein. The 63-kDa protein catalyzes DNA-dependent dTTP hydrolysis and has helicase activity; both specific activities are similar to those determined for the 56-kDa protein. The 63-kDa protein efficiently synthesizes sequence-specific di-, tri-, and tetraribonucleotides and stimulates the elongation of tetraribonucleotides by T7 DNA polymerase. Although the 56-kDa protein alone lacks primase activity, it enhances the primase activity of the 63-kDa protein 4-fold. This stimulation can be accounted for by a similar increase in the amount of primers synthesized by the 63-kDa protein in the presence of the 56-kDa protein.





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