Operator sequences of bacteriophages P22 and 21

We have determined the sequences of the left and right operators of bacteriophages P22 and 21. The corresponding operators of the two phages have nearly identical sequences, thus explaining how the repressor of each phage recognizes the operators of the other. Experiments probing the binding of repressor and operator show that each operator contains three repressor binding sites. The repressor binding sites are 18 base-pair, partially symmetric sequences. The dispersed symmetric sequence A.T.AAG.…CTT.A.T is highly conserved among the 12 repressor binding sites of the two phages. Four virulent mutations have been sequenced; all of them alter bases in the conserved sequence.