Chapter 17: Gene Regulation in Bacteria and Bacteriophages

Gene Regulation in Bacteria and Bacteriophages begins by distinguishing between constitutive housekeeping genes and regulated genes, introducing the fundamental concept of the operon as a cluster of genes transcribed together into a polycistronic mRNA. The text extensively details the negative and positive control mechanisms of the lac operon in E. coli, explaining how the Lac repressor binds to the operator to block transcription in the absence of lactose, and how the inducer allolactose causes an allosteric shift to release the repressor. It also explores genetic mutations such as lacOc and lacI- that lead to constitutive expression, establishing the concepts of cis-dominance and trans-dominance. The chapter further elaborates on catabolite repression, describing how glucose levels influence cyclic AMP concentrations and the binding of the catabolite activator protein (CAP) to the promoter to regulate transcription efficiency. The discussion then shifts to repressible systems using the trp operon as a model, where the presence of tryptophan activates an aporepressor to inhibit transcription. A key concept covered is attenuation, a regulatory mechanism dependent on the close coupling of transcription and translation, where the formation of specific RNA secondary structures in the leader region determines whether transcription terminates or proceeds based on amino acid availability. The text also analyzes the ara operon, highlighting the unique dual role of the AraC protein as both a repressor and an activator that mediates DNA looping. Finally, the chapter investigates the complex genetic switch in bacteriophage lambda, detailing the competition between the cI repressor and the Cro protein which determines the decision between the lytic and lysogenic pathways, and the role of antiterminator proteins like N and Q in viral gene expression.