Department of Biochemistry and Molecular Biology, 301 University Boulevard, University of Texas Medical Branch, Galveston, TX 77555-0647, USA.
Cryo-electron microscopy (cryo-EM) studies of the bacteriophage phi29 DNA packaging motor have delineated the relative positions and molecular boundaries of the 12-fold symmetric head-tail connector, the 5-fold symmetric prohead RNA (pRNA), the ATPase that provides the energy for packaging, and the procapsid. Reconstructions, assuming 5-fold symmetry, were determined for proheads with 174-base, 120-base, and 71-base pRNA; proheads lacking pRNA; proheads with ATPase bound; and proheads in which the packaging motor was missing the connector. These structures are consistent with pRNA and ATPase forming a pentameric motor component around the unique vertex of proheads. They suggest an assembly pathway for the packaging motor and a mechanism for DNA translocation into empty proheads.