Chapter 16: Regulation of Gene Expression in Bacteria

Efficient genetic expression is largely achieved through controlling the initiation of transcription, utilizing both negative control, where gene expression occurs unless actively shut off by a regulator molecule, and positive control, where transcription only occurs if a regulator molecule directly stimulates RNA production. Bacterial genes encoding related functions are often organized into clusters called operons, which are coordinately regulated and can be classified as either inducible or repressible. The classic lac operon system illustrates inducible negative control of lactose metabolism; the lacI gene produces a repressor protein that normally binds to the operator region, preventing RNA polymerase from transcribing the structural genes (lacZ,lacY,lacA). When the inducer, lactose (specifically allolactose), is present, it binds to the repressor, causing an allosteric conformational change that releases the repressor from the DNA, thus permitting transcription of the genes into a single polycistronic mRNA. Additionally, the lac operon is subject to catabolite repression, a form of positive control that ensures glucose is utilized preferentially over lactose. Maximal transcription requires low glucose, which leads to high cyclic AMP (cAMP) levels; cAMP forms a complex with the Catabolite-Activating Protein (CAP), and this cAMP–CAP complex binds to the promoter, facilitating RNA polymerase binding. Conversely, the tryptophan (trp) operon exemplifies a repressible negative control system used for amino acid biosynthesis. When tryptophan is abundant, it acts as a corepressor, binding to a normally inactive repressor to form a complex that successfully blocks the operator and halts transcription. Furthermore, bacteria employ regulatory mechanisms involving RNA: Attenuation in the trp operon fine-tunes repression via premature transcription termination dictated by the formation of alternative secondary mRNA structures (terminator or antiterminator hairpins) in the leader sequence, depending on tryptophan availability. Riboswitches are sequences in the 5' untranslated region (UTR) of mRNAs that bind small ligands (metabolites) at an aptamer site, causing a conformational change in the expression platform that often induces a transcription terminator structure. Finally, small noncoding RNAs (sRNAs) modulate gene expression by binding to target mRNAs, which can either inhibit translation by masking the ribosome-binding site (RBS) or enhance translation by preventing secondary structures from obscuring the RBS. Bacterial collective behaviors, such as virulence and biofilm formation, are also coordinated through quorum sensing (QS), a system where bacteria use autoinducers to sense population density and regulate gene expression across the group.