17 Molecular Characteristics of the Regulatory Switch in Phages 434 and λ

OVERVIEW The dimeric repressor and Cro proteins of temperate bacteriophages λ and 434 recognize operator sites with approximate twofold symmetry. Each protein monomer is anchored on its half-site by a set of positioning contacts that include hydrogen bonds to DNA phosphates. Base pairs in the outer parts of the operator are specified by interactions with side chains on the recognition helix of the helix-turn-helix motif. Base pairs near the center of the operator are specified in λ by a second recognition element, the amino-terminal arm, and in 434 by the effects of sequence on DNA conformability. Differences between repressor and Cro that are important for differential recognition are distributed; they are not restricted to one or a few side chains that contact bases directly. INTRODUCTION The temperate bacteriophages related to λ can switch from lysogeny to lytic growth in response to induction by UV irradiation and other stimuli (Lwoff 1953). Two proteins, repressor and Cro, and two major operators, O R and O L , govern the immediate molecular mechanism of this decision (see references in Johnson et al. 1981; Ptashne 1986). Each operator has three sites, all of which bind both proteins. A critical characteristic of O R (Fig. 1) is that repressor binds most tightly to site O R 1 and least tightly to site O R 3, whereas Cro has opposite relative affinities. Occupancy of O R 1 represses transcription from promoter P R ; occupancy of O R 3 represses transcription from P RM . Thus, the presence of repressor maintains lysogeny by inactivating lytic functions transcribed from P R and controls its...