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λ Repressor Function and Structure

Mark Ptashne


In this paper I will summarize our understanding of the molecular basis of the following phenomena involving the function and synthesis of the bacteriophage λ repressor.

In its classical role, repressor turns off transcription of other phage genes at the promoters PL and PR, thereby maintaining the phage chromosome as prophage in a lysogenic bacterium. These phage genes are thus heterogenously regulated by repressor (Fig. 1).

In lysogens the repressor gene (CI) is autogenously regulated. Repressor activates CI transcription at low concentrations and represses it at high concentrations. The promoter for CI transcription in lysogens is called PRM (promoter for repressor maintenance) (Fig. 1).

Upon infection of nonlysogens (i.e., in the absence of repressor), repressor is synthesized in a mode different from that in lysogens. In this case, CI is transcribed under the direction of regulatory proteins encoded by phage genes CII and CIII. The promoter used is called PRE (promoter for repressor establishment) (Fig. 1). In a cell destined to become a lysogen, this burst of repressor turns off transcription of CII and CIII and of other phage genes as it turns on transcription of CI from PRM.

Phage λ sometimes grows lytically in nonlysogens. In this case, synthesis of a second repressor encoded by the cro (or tof) gene is required. The cro product represses early genes, including cro, by blocking transcription beginning at PL and PR. In addition, the cro protein represses transcription of CI originating at PRM.

Treatment of lysogens...

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