λ Repressor Function and Structure

Abstract

INTRODUCTION 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 P L and P R , 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 P RM ( p romoter for r epressor m aintenance) (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 P RE ( p romoter for r epressor e stablishment) (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 P RM . 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 P L and P R . In addition, the cro protein represses transcription of CI originating at P RM . Treatment of lysogens...