GENETIC-BASIS OF BACTERIOPHAGE HK97 PROHEAD ASSEMBLY

We report studies to determine which bacteriophage genes are required for assembly of phage HK97 proheads and what roles they play. We ident ify the gene encoding the major capsid protein of phage HK97 and repor t its DNA sequence, together with the DNA sequences of the two genes i mmediately upstream from it. When the capsid protein is expressed from a plasmid in the absence of other phage-encoded proteins, it assemble s, with good efficiency and accuracy into prohead-like structures comp osed of the unprocessed 42 kDa capsid protein. No separately encoded s caffolding protein is required for this assembly If the 25 kDa product of the next gene upstream is co-expressed with the capsid protein, th e prohead structures that are produced undergo the normal morphogeneti c cleavage, which removes 102 amino acids from the N terminus of each subunit, leaving 31 kDa subunits. The 25 kDa protein is therefore prob ably a phage-encoded protease. The third gene, upstream from the prote ase gene, encodes the portal protein. Presence of the portal protein i s not required for assembly of the capsid protein. Analysis of the phe notypes of four single amino acid-substitution mutants in the capsid-p rotein gene leads to several insights into the functions of the capsid protein and its interactions with the putative protease.