Superinfection in bacteriophage S13 and determination of the number of bacteriophage particles which can function in an infected cell.

Bacteriophage S13 shows exclusion of superinfecting homologous phage, but the exclusion is only partial. The superinfecting phage can form infectious replicative form deoxyribonucleic acid (RF), can direct protein synthesis, and can form progeny particles even at a superinfection time as late as 60 min after the first infection. Exclusion is also only partial for the closely related phage phiX174. Seven min after the first infection, the exclusion mechanism begins to operate, requiring continuous phage-specified protein synthesis. The gene A protein (required for synthesis of progeny RF) appears to be involved in the exclusion mechanism. In superinfection experiments, it was found that at least 40 phage particles per cell can replicate and can carry out protein synthesis, though the number of sites for binding of RF to the membrane is only about 15 per cell. The results suggest that attachment of RF to a binding site is not required for protein synthesis. Evidence is presented that non-attached parental RF can serve as a template for single-stranded deoxyribonucleic acid synthesis.