Chapter 6 Prophage Insertion and Excision

Abstract

Cells which are immune to infection by lambda and produce phage on induction are called lysogens; they are descended from a cell in which an infecting λ phage particle entered the lysogenic pathway. The lysogenic pathway can be divided into two distinct steps. The first step, which requires among other things synthesis of repressor, results in repression of phage gene expression and of autonomous phage replication (see Chapters 9, 11, and 12). The second step is the attachment of the repressed phage chromosome to the chromosome of the bacterial host (Lederberg and Lederberg, 1953; Wollman, 1953). These two steps, repression and attachment, were clearly shown to be under independent genetic control by the analysis of a deletion mutant, λb2 (Kellenberger et al., 1961; Zichichi and Kellenberger, 1963). A b2 mutant is able to repress its functions, but infected male cells do not transfer the repressed DNA to female recipients in bacterial mating. This indicates that λb2 DNA fails to attach to the bacterial chromosome. Cells that survive infection by λb2 do not give rise to clones of lysogenic descendants. Instead, the unattached repressed phage chromosome fails to replicate and is transmitted to only one daughter at each cell division. This response is known as abortive lysogeny.

The analysis of λb2 suggested that chromosomal attachment is essential for the establishment of stable lysogeny. It is now known (see below) that chromosomal attachment is achieved by the insertion of the prophage into the bacterial DNA. In this chapter, we shall discuss how...