Gene Control by the Lambda Phage Repressor

Abstract

The DNA of coliphage λ encodes about 50 genes. In the lysogenic state, the phage DNA molecule is integrated into the host chromosome, and most of the phage genes are turned off by the product of the phage cI gene, the so-called λ phage repressor. Upon induction of lysogens, a consequence of inactivation of repressor, the phage chromosome detaches from the host chromosome and ordinary lytic phage growth commences. The mechanism of attachment and excision of the phage chromosome is well known: A single reciprocal recombination, which normally occurs at a unique site on both the phage and host chromosome, results in integration, and excision is a reversal of this process. These remarkable recombination events are catalyzed by proteins coded by the phage and by the host, and they are discussed in detail by Howard Nash at this colloquium. The essential point to grasp here is that integration and excision do not affect gene functioning in λ. Rather, enzymes which regulate these processes are strictly controlled by regulatory proteins coded by the phage, in particular, the repressor.