The Bacteriophage 434 Right Operator Roles of OR1, OR2 and OR3

Abstract

Lysogenic induction of bacteriophage λ is controlled by the action of the phage repressor and Cro proteins at the phage right operator (OR). 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 OR 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 (OR1, OR2 and OR3) 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 λ 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 OR1, whereas binding of λ Cro to OR1 and OR2 contribute equally to repression. (2) The 434 cI message, unlike that of λ, has a recognizable homology to the Shine-Dalgarno ribosome binding site. (3) Occupancy of OR3 by repressor may be somewhat greater in a 434 lysogen than in a λ lysogen. (4) The 434 repressor probably activates transcription when bound at OR2 by contacting RNA polymerase, as does λ 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 λ, P22 and 434: apposition of an acidic surface to a particular part of RNA polymerase.