Chapter 31: Gene Expression Control in Prokaryotes

Gene Expression Control in Prokaryotes begins by exploring the structural basis of protein-DNA interactions, highlighting how regulatory proteins often utilize specific motifs, such as the helix-turn-helix, to recognize and bind to symmetrical DNA sequences within the major groove. A central theme is the operon model, exemplified by the lactose (lac) operon in Escherichia coli, where gene clusters encoding related metabolic enzymes are transcribed together as a single polycistronic mRNA. The text details the mechanics of negative regulation, where the lac repressor tetramer binds to the operator site to block RNA polymerase, a repression that is only relieved when the inducer allolactose triggers a conformational change that dissociates the repressor from the DNA. Conversely, the purine repressor illustrates a system where DNA binding and transcriptional inhibition occur only in the presence of a corepressor. The discussion extends to positive regulation and catabolite repression, explaining how the Catabolite Activator Protein (CAP) complexes with cyclic AMP (cAMP) under low-glucose conditions to energetically assist RNA polymerase recruitment and stimulate gene expression. The chapter also analyzes the complex genetic circuitry of bacteriophage lambda, describing the bistable genetic switch between lytic and lysogenic pathways, which is determined by the competitive interplay and feedback loops between the lambda repressor and the Cro protein. Furthermore, the text addresses intercellular communication through quorum sensing, a process where bacterial populations detect cell density via chemical autoinducers like acyl-homoserine lactones (AHLs) to coordinate collective behaviors such as bioluminescence and biofilm formation. Finally, the summary covers posttranscriptional regulation via attenuation, a mechanism observed in amino acid biosynthetic operons like tryptophan, where the secondary structure of the mRNA leader sequence serves as a sensor for translational speed and amino acid availability, determining whether transcription proceeds or terminates prematurely.