Molecular mechanisms in bacteriophage T7 procapsid assembly, maturation, and DNA containment.

Bacteriophage T7 is a double-stranded DNA bacteriophage that has attracted particular interest in studies of gene expression and regulation and of morphogenesis, as well as in biotechnological applications of expression vectors and phage display. We report here studies of T7 capsid assembly by cryoelectron microscopy and image analysis. T7 follows the canonical pathway of first forming a procapsid that converts into the mature capsid, but with some novel variations.

A second symmetry mismatch at the portal vertex of bacteriophage T7: 8-fold symmetry in the procapsid core.

Like other bacteriophages, T7 has a singular vertex that is the site of a symmetry mismatch involving the portal/connector protein, a 12-fold ring at the vertex site which is also a 5-fold axis for the icosahedral capsid. In the mature virion, a 6-fold-symmetric tail extends outwards from the connector. T7 also has a cylindrical "core" that assembles on the inner surface of the connector during procapsid formation, is retained in the mature virion, and is required for infectivity.