Head maturation pathway of bacteriophages T4 and T2. III. Isolation and characterization of particles produced by mutants in gene 17.

We have isolated and characterized two types of particles produced in comparable amounts by mutants in gene 17: the empty large particle and the empty small particle. Dimensions, morphology, stability, and protein composition of the empty large particle are very similar to those of the capsids or empty heads of mature phage. The other type of particle (empty small particle) is very similar in dimensions and stability to the prehead, but differs in that it is composed of processed proteins (gp23, gp24, IpIII). Structural analysis has shown that the protein subunits of the empty small particles are arranged in an unexpanded type of lattice (11.2 to 11.3 nm), whereas the empty large particles have an expanded lattice (13 nm). The characterization of the empty small particle as being composed of cleaved proteins, but still unexpanded, shows that the expansion of the T4 head shell is not necessarily linked to the cleavage of the structural proteins.