2 Genetic Studies of RNA Phages

Abstract

INTRODUCTION
The feature of the RNA phages which has made them such important objects of study is the small size of their genomes. Reports of the molecular weight of the phage RNA varies from 1.0 to 1.5 × 10⁶ (see Chapter 1). Thus the maximum number of amino acids which could be coded for by the phage genome is 1000–1500, which in turn limits the number of cistrons in the genome to probably three or four. This also suggests that all of the gene functions would be indispensable for the growth of the phage.

The discovery of RNA phages by Loeb and Zinder (1961) came just at the time when the concept of conditionally lethal mutations was being established by the work on phage T4 (Benzer and Champe 1962; Epstein et al. 1963), phage lambda (Campbell 1961), and on alkaline phosphatase (Garen and Siddiqi 1962). Isolation and characterization of conditional-lethal mutants of the RNA phages were started soon after the initial characterization of the phage and served to identify all the genetic functions of the phage (Zinder and Cooper 1964).

Complementation analyses and physiological analyses of the amber and temperature-sensitive mutants of the RNA phages have established the existence of the three cistrons shown in Table 2.1 (but see below) (Horiuchi, Lodish and Zinder 1966; Gussin 1966; Horiuchi and Matsuhashi 1970).

The in vivo synthesis of phage-specific proteins was detected by inhibiting host syntheses with actinomycin (Oeschger and Nathans 1966) or rifampicin (Fromageot and Zinder 1968), and these proteins...