The bacteriophage 434 right operator. Roles of O(R)1, O(R)2 and O(R)3.

Lysogenic induction of bacteriophage lambda is controlled by the action of the phage repressor and Cro proteins at the phage right operator (O(R)). This study examines the roles of the repressor and Cro proteins of the related phage 434. The start sites of transcription of the divergently oriented promoters in the 434 O(R) region, PR and PRM, were mapped, and the effects of 434 repressor and Cro on promoter activity were assessed using promoter fusions to lacZ. The effects of repressor or Cro bound to each of the operator subsites (O(R)1, O(R)2 and O(R)3) were assessed by examining regulation in the presence of operator mutations. The binding of Cro to a 434 operator was probed by an ethylation interference experiment which, together with other data, indicates that 434 Cro and repressor probably turn off transcription by blocking binding of RNA polymerase to promoter sequences. In general, the 434 and lambda right operators are controlled in a similar fashion, but differences in detail were also encountered: (1) 434 Cro represses transcription from PR primarily by binding to O(R)1, whereas binding of lambda Cro to O(R)1 and O(R)2 contribute equally to repression. (2) The 434 cI message, unlike that of lambda, has a recognizable homology to the Shine-Dalgarno ribosome binding site. (3) Occupancy of O(R)3 by repressor may be somewhat greater in a 434 lysogen than in a lambda lysogen. (4) The 434 repressor probably activates transcription when bound at O(R)2 by contacting RNA polymerase, as does lambda repressor, but also by influencing competition between PR and PRM. An analysis of the six right operator systems for which data are available indicates that all six repressors may employ the mechanism of transcriptional activation first described for lambda, P22 and 434: apposition of an acidic surface to a particular part of RNA polymerase.