3 Proteins of the RNA Phages

Abstract

INTRODUCTION The phage RNAs specify three proteins, namely, the coat, maturation, and a subunit of the replicase. In general, the mature virion contains 180 copies of the coat protein and a single copy of the maturation protein. However in Q β , the virion may contain a third component, the so-called “read-through” protein. The synthetase polypeptide assembles with three host polypeptides to form an active replicase inside the infected cell and is not incorporated into the virus particles. In this chapter we will discuss the structure and possible functions of the coat protein, the maturation protein, the phage-specified polypeptide of the replicase, and the origin and possible function of the “read-through” protein.* THE COAT PROTEINS Classification of RNA Phages The RNA bacteriophages were first isolated and characterized by Loeb and Zinder (1961). Since then, many RNA coliphages have been independently isolated from sewage effluents throughout the world. Despite the different geographical sources, all the RNA bacteriophages can be classified into three main groups based on the chemical and immunological properties of their coat proteins (Watanabe et al. 1967; Nishihara et al. 1969). Some of the coat proteins from group I phages have been sequenced, such as f2 (Weber and Konigsberg, 1967), R17 (Weber 1967), fr (Wittman-Liebold and Wittman, 1967), MS2 (Lin, Tsung and Fraenkel-Conrat 1967; Vandekerckhove, Francq and Van Montagu 1969) and ZR (Nishihara, Nozu and Okada 1970). The coat protein from Q β phage, a representative of group III, has also been sequenced (Maita and Konigsberg 1971). However, only the amino acid...