The functions of four T4 genes, 48, 54, 19 and 3, are required for the formation of the complete tail core on the phage baseplate. Cells infected with amber mutants defective in genes 48, 54 or 19 accumulate free phage baseplates. Viable phage are formed in pairwise mixtures of extracts of these mutant-infected cells, indicating that core formation is proceeding in vitro. The baseplates have been examined with the electron microscope and by centrifugation and are indistinguishable by these criteria except that baseplates from gene 48- preparations form dimers between their inner faces. In vitro complementation tests with isolated baseplates show that the three classes are functionally distinct, and that the products of genes 48, 54 and 19 interact with baseplates sequentially in the order P48:P54:P19, and only in that order.
Genes 48 and 54 appear to specify proteins required for the initiation of tail core polymerization. Experiments with temperature-sensitive mutants in gene 19 indicate that this gene specifies the major structural subunit of the tail core. The product of gene 3 is not required for the formation of the core, but for the formation of the sheath-core terminus. P3 apparently forms or acts on the terminal annulus of the core to make it a substrate for sheath attachment.
A pathway for phage core and sheath assembly is presented in which each gene product interacts only with a specific large precursor structure. A property of this pathway is that the formation of aberrant structures is avoided.