T7-directed protein synthesis

Abstract

The protein composition of purified T7 phage particles, the patterns of protein synthesis after infection with wild-type and amber mutants of T7, and molecular weights of the individual polypeptide chains have been examined by electrophoresis on polyacrylamide gels containing SDS at neutral pH. Purified preparations of T7 particles contain 9 resolvable proteins, which together account for approximately 43% of the coding capacity of T7 DNA. The protein composition of partly assembled phage particles has also been examined, and the last step in the assembly of infective T7 particles identified. Comparing the protein compositions with electron micrographs of the particles permits the identification of a single protein as the major, if not the only, component of the tail. After infection of the host, T7 directs the synthesis of approximately 25–30 proteins, which together account for essentially all of the coding capacity of T7 DNA. From the time course of protein synthesis after infection, perhaps three classes of proteins can be distinguished, earliest, early, and late. Mutants in gene 1 are the only ones found so far which affect the control of protein synthesis. In the absence of gene 1 function, only the three earliest proteins are made. DNA synthesis is not required in order to synthesize late proteins, but they are made in much larger amounts when the synthesis of phage DNA is normal. So far, proteins have been identified for 12 of the 19 genes known for T7, including the genes for 6 of the 9 proteins of the T7 particle. Amber peptides can be identified in mutants from several genes, and correlation of their length with the position of the mutation permits determination of the direction of translation relative to the genetic map. This information has been combined with data available in the literature to orient the T7 DNA molecule, the direction of transcription and translation, and the genetic map relative to each other.