The Proteins of Bacteriophage M13 by Timothy J. Henry and David Pratt

-Particles of the small filamentous coliphage M13 contain not only the major coat protein, which is the product of phage gene 8, but also a minor coat protein, the A protein, which is the product of gene 3. The A protein has a molecular weight of approximately 70,000 daltons, is present in one copy per virion, and is responsible for phage attachment to host cells. Also associated with purified M13 particles is a minor quantity of very small proteinaceous material, but its origin as a phage-coded product has not been demonstrated. At least five phage-specific proteins, including the two coat proteins, are present in appreciable quantities in M13-infected cells. The principal phage protein synthesized is the product of gene 5, which is responsible for phage singlestranded DNA synthesis. This protein has a molecular weight of about 8,000 daltons. Its precise function in DNA synthesis is not yet known. Phage proteins are synthesized at nearly normal rates in cells in which replication of phage double-stranded DNA is blocked by gene 2 mutations. This result suggests that the initial double-stranded DNA molecule serves as the principal template, perhaps the only template, for phage messenger RNA synthesis. Introduction.-M13 is a male-specific bacteriophage containing 2 X 106 daltons of single-stranded DNA.' We have been interested in determining both the number of proteins in the filamentous M13 virion and the total number of phagespecific proteins formed within infected Escherichia coli cells. Based on genetic evidence,2' I it appears that the virions should contain not only the major coat protein4' I but also a minor protein for phage attachment to the F pili of the host cells. Based on the genome size, the total number of proteins encoded by M13 should be about ten. This is in fair agreement with the presently known number of phage genes (eight) as defined by conditional lethal mutants.2' 6 In this communication, we report on polyacrylamide gel studies of virion and intracellular M13 proteins, with particular emphasis on establishing which proteins are coded by known phage genes. Materials and Methods.-The bacterial strains, bacteriophages, cultural techniques, and media routinely used for M13 have been described previously.6-8 Radioactive amino acids were purchased from the New England Nuclear Corp. M13 virions, labeled either singly with H8-leucine or doubly with H8-valine plus C14-arginine, were prepared as described in the figure legends. The labeled virions were purified by three cycles of differential centrifugation and banding in a CsCl density gradient. Virion proteins were extracted and prepared for gel electrophoresis using the procedures described by Nathans et al.9 for proteins from sonicated cells. Labeled intracellular phage proteins were prepared according to the general procedure developed by Ptashne,'0 involving ultraviolet (UV)* irradiation of the host cells before infection to suppress cellular protein synthesis. The UV-sensitive subacterial strain 159F+ was grown to a cell density of 4 X 108/ml in M9 medium containing 0.1% casamino acids and 0.2% glucose. The culture was irradiated in a Petri dish cover at a depth of