In addition to one molecule of positive-strand RNA, each virion contains 180 copies of the coat protein and one copy of the maturation, or A, protein. The group III phages contain in addition about 12 copies of the read-through protein in their capsid. For this reason, the protein shell of the single-stranded (ss) RNA phages is not isomeric like other icosahedral viruses such as poliovirus or satellite tobacco necrosis virus. The diameter of the phage is 26 nm, and the protein shell is about 2nm wide (Fig. 2). The icosahedral shell has a T = 3 surface lattice. Crystals of phage MS2, GA, fr and Q/? have been obtained. The coat protein structure of MS2 has been solved to 2.7 A resolution by x-ray diffraction. Unfortunately, the RNA and the A protein are not seen in the electron
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