Supercoiled DNA wraps around the bacteriophage phi 29 head-tail connector.

Supercoiled pBR322 DNA wraps around the outside of the isolated Bacillus subtilis bacteriophage phi 29 head-tail connector, the crux of the DNA packaging machine of the viral precursor capsid or prohead. The contour length of the supercoiled DNA, determined by EM, decreased by approximately 180 base pairs for each connector bound. Mass and radial density determinations by scanning transmission EM showed that the increased mass of the connector-DNA complex relative to the connector alone was equivalent to approximately 170 base pairs of DNA and was located around the outside of the connector. Topoisomerase I treatment of the complexes followed by deproteinization suggested that supercoils were restrained by the connectors. Connectors bound linear and open-circular plasmid DNAs inefficiently but were not wrapped by these DNAs. The wrapping of supercoiled DNA around the isolated phi 29 connector is hypothesized to reflect the initiation phase of the normal process of DNA packaging. Packaging substrates would be supercoiled, wrapped by the connector, linearized, and translocated by rotation of the connector relative to the viral capsid with the aid of ATP hydrolysis.