DNA packaging and the pathway of bacteriophage T4 head assembly.

Abstract

A cold-sensitive mutation in the structural gene for a minor phage T4 capsid protein (p20) leads to formation of heads containing p20 and cleaved head proteins and empty of DNA. Such heads can be filled with DNA and converted to active phages in vivo upon shift to high temperature. Apparently p20 has 2 distinct roles in head assembly: 1st, in construction of the prehead shell (blocked by ts and am mutation) and, 2nd, in DNA packaging (blocked by cs mutation). The latter function is closely associated with gene 17 product, previously known to be required for DNA packaging. Temperature shift studies of cs-ts double mutants and other observations allow determination of phage functions required for DNA packaging. Contrary to previous proposals, T4 DNA packaging is not directly coupled to and can follow DNA synthesis, protein cleavage, prehead core removal and gene 21-mediated cleavage-induced increase in head volume. Apparently, an altered head assembly pathway exists and DNA packaging is probably initiated by DNA-capsid (p20) interaction.