Internal proteins of bacteriophage T4D: Their characterization and relation to head structure and assembly

Three basic proteins of low molecular weight (about 8000, 10,000 and 18,000) were isolated from the T4D phage particle. Many molecules of each protein are located within the phage head, possibly in association with the DNA, and together with the proteins which form the head membrane comprise most of the head structural protein. The purified internal proteins were characterized by physicochemical and immunological techniques; a radio-immunoassay allowed measurement of their synthesis in phage infected bacteria. Each internal protein is synthesized at both early and late times after infection. Their structural genes are present in the phage genome, but do not appear to be among the known amber mutant-containing genes of T4D. No evidence was found to suggest that the internal proteins are formed from a common precursor molecule, nor are their origins related to those of the internal peptides; however, one of the internal proteins may be altered before its incorporation into the phage. Pulse-chase experiments with two of these proteins show that they are incorporated into certain defective T4D heads. Whether or not they are incorporated appears to depend on the degree of completion of these heads, perhaps with respect to DNA packaging.