Architecture of the bacteriophage T4 primosome: electron microscopy studies of helicase (gp41) and primase (gp61).

Replication of DNA requires helicase and primase activities as part of a primosome assembly. In bacteriophage T4, helicase and primase are separate polypeptides for which little structural information is available and whose mechanism of association within the primosome is not yet understood. Three-dimensional structural information is provided here by means of reconstructions from electron microscopic images. Structures have been calculated for complexes of each of these proteins with ssDNA in the presence of MgATPgammaS. Both the helicase (gp41) and primase (gp61) complexes are asymmetric hexagonal rings. The gp41 structure suggests two distinct forms that have been termed "open" and "closed." The gp61 structure is clearly a six-membered ring, which may be a trimer of dimers or a traditional hexamer of monomers. This structure provides conclusive evidence for an oligomeric primase-to-ssDNA stoichiometry of 6:1.