Isolation and characterization of a portal protein-DNA complex from dsDNA bacteriophage

A portal protein-DNA complex was isolated from bacteriophage disrupted in 70% acetic acid. This treatment dissociated most of the phage structural proteins, while the portal protein ring remained attached to the highly condensed naked DNA. In the absence of a headful of DNA, the portal ring was not protected. Purification of the portal ring-DNA complex by density gradient centrifugation showed that a dsDNA fragment of about 40 bp was protected from DNase digestion by this association, suggesting it was internalized within the 140 A long, 20-40 A in diameter T4 apical channel of the ring dodecamer. A portal ring-DNA complex could be isolated from a number of phages (T4, T7, Phi29), and the end(s) of phage T7 DNA was (were) found to be preferentially associated with the ring complex.