Identification of two new capsid proteins in bacteriophage M13

Abstract

MI 3 is a male-specific filamentous coliphage , closely related to the phages fd and fl (reviewed [ 1,2]). The virion is composed of a circular singlestranded DNA (mol. wt 2 X 106) encapsulated in a tubular protein coat of about 850 nm long and 6 nm wide. Up to now it has generally been assumed that the virion coat is composed of two proteins only. The major coat protein (mol. wt 5200) encoded by gene VIII is present in about 2400 copies and accounts for virtually all of the protein mass of the virion coat. The minor coat protein (mol. wt 42 600) which is encoded by gene III is present in 4-5 copies only which are located at one tip of the filament. Since there have been reports [3-51 which suggest that the filamentous phages may contain an additional protein component, we have investigated the protein constituentsof Ml3 virions in more detail. Here we demonstrate that the Ml3 virion contains besides the gene III and gene VIII encoded products two additional capsid proteins. These proteins, designated C and D protein, have app. mol. wt 3500 and 11 500, respectively. We further could denlonstrate that the smallest structural protein is not the product of one of the eight well-characterized Ml 3 genes but is encoded by the hitherto unidentified gene IX [6,7].