Repressor and Cro Protein: Structure, Function, and Role in Lysogenization

Abstract

λ’s choice between lysogeny and lytic multiplication is influenced directly by the relative levels of two phage-encoded regulatory proteins: repressor (cI-gene product) and cro protein. Although the two proteins bind to the same sites on λ DNA, their effects on phage development are mutually antagonistic.

Repressor is essential for lysogenization. By binding to two phage operators, o_R and o_L, it blocks transcription initiation from two promoters, p_R and p_L. In so doing, it prevents expression of phage early genes (including the cro gene) whose products are necessary for lytic multiplication. In addition, repressor bound at o_R stimulates p_RM, the promoter that directs transcription of the cI gene of an integrated prophage. Thus, repressor promotes lysogenization in two ways: It blocks expression of lytic genes, and it positively regulates its own synthesis.

In contrast to repressor, cro protein promotes lytic multiplication, primarily by inhibiting repressor synthesis both during infection of a sensitive cell and following induction of a lysogen. These effects of cro protein are also mediated by binding to o_R and o_L. In subsequent sections one will see that the ability of repressor and cro protein to bind to the same operators and yet serve distinct physiological functions is due to (1) the existence within each operator of three repressor (and cro)-binding sites, (2) the differing relative affinities of the two proteins for each of the sites, and (3) the spatial relationship between the operator sites and the promoters p_L, p_R, and p_RM.

In this review we consider the basic...