The cI repressor is a dimeric protein from bacteriophage , with a subunit Mr of 27,000. cI repressor binds through its N-terminal sequences to operator sites with a Ka of about 3 x 10¹³ M (Figure 26.24). Two operators, on either side of the cI gene, control divergent transcriptional events from a central regulatory region-leftward (OL) and rightward (OR). Each operator contains three separate repressor-binding sites, each about 17 base pairs long. The three repressor-binding sites are homologous, but not completely so (Figure 26.26), and they are separated by spacer regions of three to seven base pairs. A fully virulent mutant (one which does not establish a lysogenic state) has at least two mutations-one in OL and one in OR.