Bacteriophage P22 mutants that alter the specificity of DNA packaging

Abstract

Bacteriophage P22 cleaves and packages the mature viral chromosome from a concatemeric precursor by sequential headfuls. Generalized transducing particles are formed when occasionally host rather than phage DNA is cut and packaged. Since infected cells contain about as much bacterial DNA as P22 DNA at late times, but most P22 particles contain only viral DNA, the packaging mechanism must be able to choose phage DNA in preference to host DNA as substrate for encapsulation. The HT mutants isolated by Schmieger (1971b, 1972) appear altered in this packaging specificity, since they form transducing particles at substantially increased frequencies. The HT mutants have now been shown to have reduced specificity for pac, the site on P22 DNA at which headful packaging normally initiates. One mutant, HT 12/4, no longer packages any phage DNA at pac, but recognizes a new site on P22 DNA. The HT 12/4 mutant is shown to make a gene 3 protein of altered specificity, suggesting that the gene 3 function may be required to recognize pac. These results indicate that recognition of pac is the mechanism by which phage DNA is packaged in preference to host DNA.