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Bacterial RNA polymerase recognizes promoters via sigma factors and transcribes operons—coordinately regulated clusters of functionally related genes. Negative control (e.g., lac repressor) and positive control (e.g., CAP-cAMP) allow rapid adaptation to nutrient availability. This organization contrasts sharply with eukaryotic gene regulation.
You already understand the basics of prokaryotic gene regulation — that bacteria control which genes are expressed and when, primarily at the level of transcription. This topic builds on that foundation by examining how bacterial transcription machinery and operon architecture work together as an integrated regulatory system, enabling bacteria to respond to their environment with remarkable speed and efficiency.
Bacterial RNA polymerase is a multi-subunit enzyme (core enzyme: α₂ββ'ω) that cannot, on its own, find the right place to start transcribing. It needs a sigma factor (σ) to recognize promoter sequences. The primary sigma factor, σ⁷⁰ in *E. coli*, directs transcription of housekeeping genes by recognizing conserved -10 and -35 promoter elements. But bacteria carry alternative sigma factors — σ³² for heat shock genes, σ⁵⁴ for nitrogen metabolism, σˢ for stationary-phase survival — that redirect the polymerase to entirely different sets of promoters. Think of sigma factors as interchangeable address labels: by swapping which sigma is loaded onto the polymerase, the cell can globally reprogram its transcriptional output in response to stress, starvation, or environmental change. This is faster than modifying individual gene regulators one by one.
The operon is the organizational unit that makes this system efficient. An operon clusters functionally related genes under a single promoter so they are transcribed together as one polycistronic mRNA. The *lac* operon is the classic example: the genes for lactose uptake (lacY) and cleavage (lacZ) are adjacent and co-transcribed, ensuring the cell never makes the transporter without the enzyme or vice versa. Regulation of this operon illustrates both major control strategies. Negative control comes from the lac repressor, a protein that binds the operator (a DNA sequence overlapping the promoter) and physically blocks RNA polymerase from transcribing. When allolactose (the inducer) binds the repressor, the repressor changes shape and falls off the DNA, allowing transcription. Positive control comes from CAP (catabolite activator protein), which binds upstream of the promoter only when complexed with cAMP — and cAMP levels are high only when glucose is absent. CAP-cAMP bends the DNA and helps recruit RNA polymerase, boosting transcription roughly 50-fold.
The interplay between these two controls creates a logical AND gate: the *lac* operon is fully expressed only when lactose is present (repressor removed) AND glucose is absent (CAP-cAMP active). This catabolite repression system ensures bacteria use the most energetically favorable carbon source first — glucose — before investing in enzymes for alternative sugars. The same regulatory logic applies across many operons: the *trp* operon uses a repressor activated by tryptophan (negative control of a biosynthetic pathway), while nitrogen-regulated operons use σ⁵⁴ and activator proteins. The common thread is that bacteria regulate transcription at the operon level to coordinate gene expression with metabolic need, a strategy that is fast, economical, and fundamentally different from the enhancer-based, single-gene regulation you will encounter in eukaryotic systems.