A Beginner’s Guide to Lambda Biology

Abstract

INTRODUCTION
Figure 1 is a portrait of bacteriophage λ. The phage particle is composed of roughly half protein and half double-stranded DNA. The size of the genome of wild-type λ phage is about 48 kb, corresponding to a mass of about 32 × 10⁶ daltons.

Extensive genetic studies have been done with this bacteriophage (see Campbell 1971). Mutations with particular phenotypes, such as the formation of clear plaques (c) rather than turbid plaques (c⁺), or other aspects of plaque appearance, as well as host-range mutations (h), played important roles in the early investigations. More recent work has made extensive use of conditionally lethal mutations, including, in particular, sus (supressor sensitive) mutations, also known as nonsense mutations. The mutational events leading to sus generate any of the three nonsense codons: UAG for the amber mutations, UAA for the ochre mutations, and UGA for the opal mutations. Most of the sus mutations in use are amber, and the symbol am is sometimes used for particular sus mutations. For work with sus mutations, two bacterial host strains are needed: one that does not allow the reproduction of the phage (nonpermissive strain), and a second in which the phage is able to reproduce (permissive strain or suppressor strain). Such suppressor strains usually contain a mutation in a tRNA gene, so that a new species of tRNA is produced that is able to give sense to an otherwise nonsense codon (i.e., to insert an amino acid at what would otherwise be a protein chain termination...