The Isometric Phage Genome: Physical Structure and Correlation with the Genetic Map

Abstract

Replication of the isometric DNA bacteriophages ϕX174 and S13 during the infective cycle is a very efficient and rapid event. Within 20 minutes an infection results in the synthesis of some 200 progeny phage particles and their release through lysis of the host cell. This process is rapid both because the viral genome is small, and therefore will be replicated rapidly, and because, in its life cycle, the phage makes extensive use of already existing bacterial proteins. Also, the viral gene product that is essential in the earliest stage of phage reproduction is needed only in small amounts.

These properties of the small DNA phages make them attractive and suitable organisms for experiments in molecular biology. Our knowledge of a number of physiological processes in the bacterial cell, such as DNA replication, has been enhanced by using these viruses as test systems. From detailed analyses of particle structure, the organization of the phage DNA, and the various gene functions coded for in the viral genome, a fairly complete picture has emerged as to how these viruses, in spite of their limited genome size, are able to multiply and how they interact with many bacterial processes.

The various functions of the virus will be reflected in the organization of its genetic material. Studies of the organization of the viral genome started some 15 years ago in the classical way—by the isolation and analysis of mutants. By means of complementation analysis, the mutants were grouped into genes, and subsequent genetic recombination...