7 Structure and Function of the Qβ RNA Replicase

Abstract

INTRODUCTION
The genetic simplicity of RNA bacteriophages necessitates nearly complete dependence on the host bacterial cell for the synthesis of viral macromolecular components. RNA replication is the single aspect of virus development likely to require a phage-coded enzyme since RNA template-dependent RNA synthesis apparently does not occur in uninfected bacteria. Early work clearly established the absence of a DNA intermediate in phage RNA synthesis (Cooper and Zinder 1962; Doi and Spiegelman 1962; Haywood and Sinsheimer 1963); thus reverse transcription, now known to be used by the RNA tumor viruses (Baltimore 1970; Temin and Mizutani 1970), was excluded. Instead, infected cells were shown to contain a phage-induced RNA-dependent RNA polymerase (Weissmann, Simon and Ochoa 1963; Haruna et al. 1963), termed an RNA “replicase” by Spiegelman and Hayashi (1963). Induction of RNA phage replicases requires the synthesis of a virus-coded protein (Lodish, Cooper and Zinder 1964; Gussin 1966; Nathans et al. 1966; Viñuela, Algranati and Ochoa 1967; Horiuchi and Matsuhashi 1970). Several groups tried to purify the replicase from E. coli infected with phages f2 or MS2 (Weissmann, Simon and Ochoa 1963; August et al. 1963), but the first real success depended on the choice of a new bacteriophage, Qβ, isolated in Japan by Watanabe (1964). Haruna and Spiegelman (1965a) partially purified the Qβ replicase, obtaining enzyme preparations strictly dependent on exogenous Qβ RNA as the template for RNA synthesis. Further refinement of the isolation procedure enabled Spiegelman et al. (1965) to accomplish the first in vitro synthesis of a fully infectious...