Evolution of the Lambdoid Phages

Abstract

The purpose of this paper is to attempt to explain the origin of bacteriophage λ. For such an explanation to make sense, we must first define what we mean by “lambda,” for there is abundant experimental evidence that the organisms that we call by this name are members of a large family of similar temperate bacteriophages that share the same genome organization and the same life-style. Furthermore, members of the lambdoid family can still undergo exchanges of genes or groups of genes, producing clearly different but, nevertheless, perfectly functional lambdoid phages.

DEFINITION OF THE LAMBDOID FAMILY
A central problem then becomes the definition of the limits of the λ family. We suggest that the criterion for inclusion should be the ability to exchange genetic information with λ by homologous recombination with λ itself. If this definition is accepted, the family comprises not only the lambdoid phages of Escherichia coli but also includes phage P22, whose normal host is Salmonella typhimurium, as well as the large family of Salmonella phages related to P22. P22 has a grossly different virion morphology and lacks some features common to the lambdoid coliphages, including the cohesive DNA ends. When one takes this broad view of family membership, such striking similarities remain among all the phages in the family that abandonment of the traditional criteria for inclusion in the family (ability to grow on E. coli and cohesiveness of the DNA ends to the DNA ends of λ itself) is amply justified. Definition of the family...