Abstract
The product of gene 16 of phage P22, P16, is a head protein. P16 does not play an essential role in phage assembly since particles formed without this protein appear normal by electron microscopy examination (Botstein et al., 1973). P16 is essential when the particle infects a cell in the following cycle of infection (Botstein et al., 1973; King et al., 1973). We have characterized a mutant of P22 carrying a temperature-sensitive allele of gene 16. This mutant has previously been referred to as P22 25-ts (Levine et al., 1970, 1972) and P22 X-ts (Bezdek and Soska, 1970, 1973). P22 16-ts behaves as an early mutant at the nonpermissive temperature. Temperature shift experiments show that P16 of the infecting virion acts within the first 10 min at 25 C and that gene 16 product is required late in the latent period for incorporation into infectious phage. Induction does not require P16 for the production of particles. Particles produced either in a P22 16-ts thermal shift-up infection or after induction of 16-ts lysogens at 41 C are missing P16 and are, therefore, defective. P16 in P22 16-ts virions formed at the permissive temperature appears to be heat labile; it is inactivated after infection at 41 C. A simple assay for defective particles based on a complementation test is described.