Cro Repressor

Cro repressor is a protein in bacteriophage that is a homodimer of 66-residue subunits folded into three -helical regions and three strands (Figure 26.28). Two of the helices, numbered 2 and 3 in the figure, are separated by a short turn, forming a helix - turn - helix motif (see here).

Cro binds to the operators O R3, O R2, and O R1 in with decreasing affinities. Cro acts both as a repressor and as an antirepressor, because it antagonizes the action of cI repressor in a very specific way (Figure 26.27). Transcription from the two promoter-operator sites takes place in opposite directions along the genome (Figure 26.23). Binding of cI to O R 1 and O R2 inhibits the transcription of Cro from its own promoter and activates the leftward transcription of cI from the promoter P RM (the M stands for "maintenance," because this is the promoter from which cI is transcribed during maintenance of lysogeny). Thus, when cI binds O R1 and O R2, the lysogenic state is favored.

The lysogenic state is broken as follows:

First, cI repressor is inactivated, and the O R sites become unoccupied. This permits transcription of cro from P R, and the Cro protein blocks further transcription of cI from P RM. At the same time, leftward transcription from P L generates the N protein, blocking transcriptional termination at the sites indicated in Figure 26.27 as t R1 and t L. Thus, the two early transcripts for Cro and N are extended to activate new genes. Leftward transcription generates Int and Xis proteins, necessary for prophage excision. Rightward transcription generates O and P, necessary for DNA replication. Subsequent events lead to transcription of late genes and completion of the phage life cycle.

See also: Transcriptional Regulation in Phage , Structure of Cro and cI Repressors