2 DISPENSABLE STRUCTURAL PROTEINS (SOC AND HOC) OF T4 PHAGE CAPSID - THEIR PURIFICATION AND PROPERTIES, ISOLATION AND CHARACTERIZATION OF DEFECTIVE-MUTANTS, AND THEIR BINDING WITH DEFECTIVE HEADS INVITRO

Two classes of T4 capsid proteins, called soc and hoc, were highly purified from capsids by a general procedure for fractionating the capsid polypeptides. Their amino acid compositions, antigenicities and states in the infected bacteria [Escherichia coli] were studied. The number of soc protein molecules (MW = 10,000) present in the capsid is approximately equivalent to the number of P23* molecules, another major protein. The number of hoc protein molecules (MW = 40,000) is lower, being .apprx. 1/8 of that of P23* molecules. EM observations of antibody-bound capsids suggest that these proteins are distributed over the entire surface of the heads. Mutant phages missing soc and/or hoc proteins were isolated; these mutants are viable and the proteins are non-essential for phage growth. These mutants have normal morphology but are different from the wild type in several physico-chemical properties. The soc-defective phages are inactivated at pH 10.6, although the wild-type phages are resistant. The mutant phages are heavier in density than the wild type, as shown by CsCl density-gradient equilibrium centrifugations. A gene located between genes 39 and 56 is involved in the production of the soc protein. Binding reactions in vitro of a large number of soc or hoc molecules to defective heads were demonstrated, which might mimic the maturation of the heads in vivo. The nature of these association reactions was investigated quantitatively by using the specific characteristics of the mutants.