The Lysis-Lysogeny Decision of Bacteriophage 933W: a 933W Repressor-Mediated Long-Distance Loop Has No Role in Regulating 933W P RM Activity

Abstract

ABSTRACT Our data show that unlike bacteriophage λ, repressor bound at O L of bacteriophage 933W has no role in regulation of 933W repressor occupancy of 933W O R 3 or the transcriptional activity of 933W P RM . This finding suggests that a cooperative long-range loop between repressor tetramers bound at O R and O L does not form in bacteriophage 933W. Nonetheless, 933W forms lysogens, and 933W prophage display a threshold response to UV induction similar to related lambdoid phages. Hence, the long-range loop thought to be important for constructing a threshold response in lambdoid bacteriophages is dispensable. The lack of a loop requires bacteriophage 933W to use a novel strategy in regulating its lysis-lysogeny decisions. As part of this strategy, the difference between the repressor concentrations needed to bind O R 2 and activate 933W P RM transcription or bind O R 3 and repress transcription from P RM is <2-fold. Consequently, P RM is never fully activated, reaching only ∼25% of the maximum possible level of repressor-dependent activation before repressor-mediated repression occurs. The 933W repressor also apparently does not bind cooperatively to the individual sites in O R and O L . This scenario explains how, in the absence of DNA looping, bacteriophage 933W displays a threshold effect in response to DNA damage and suggests how 933W lysogens behave as “hair triggers” with spontaneous induction occurring to a greater extent in this phage than in other lambdoid phages.