The
opdA gene of Salmonella typhimurium encodes an endoprotease,
oligopeptidase A (
OpdA). Strains carrying
opdA mutations were deficient as hosts for phage P22. P22 and the closely related phages L and A3 formed tiny plaques on an
opdA host. Salmonella phages 9NA, KB1, and ES18.h1 were not affected by
opdA mutations. Although
opdA strains displayed normal doubling times and were infected by P22 as efficiently as opdA+ strains, the burst size of infectious particles from an
opdA host was less than 1/10 of that from an opdA+ host. This decrease resulted from a reduced efficiency of plating of particles from an
opdA infection. In the absence of a functional
opdA gene, most of the P22 particles are defective. To identify the target of
OpdA action, P22 mutants which formed plaques larger than wild-type plaques on an
opdA mutant lawn were isolated. Marker rescue experiments using cloned fragments of P22
DNA localized these mutations to a 1-kb fragment. The nucleotide sequence of this fragment and a contiguous region (including all of both P22 gene 7 and gene 14) was determined. The mutations leading to
opdA independence affected the region of gene 7 coding for the amino terminus of gp7, a
protein required for
DNA injection by the phage. Comparison of the nucleotide sequence with the N-terminal amino acid sequence of gp7 suggested that a 20-amino-acid
peptide is removed from gp7 during phage development. Further experiments showed that this processing was
opdA dependent and rapid (half-life, less than 2 min) and occurred in the absence of other phage
proteins. The
opdA-independent mutations lead to mutant forms of gp7 which function without processing.