Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 18.1 Bacteria Often Respond to Environmental Change by Regulating Transcription
Bacteria adapt to their environments by selectively expressing genes needed for survival. They control gene expression mainly at the level of transcription, responding quickly to environmental changes through regulatory mechanisms such as operons.
Regulation of Metabolic Pathways
- Cells can rapidly adjust enzyme activity (feedback inhibition) or regulate enzyme production by controlling gene expression.
- Feedback inhibition: immediate response; excess product inhibits enzyme activity.
- Gene regulation: longer-term; genes coding for enzymes can be turned on or off to control production.
Operons: Coordinated Control of Gene Expression
An operon is a segment of DNA containing a cluster of genes regulated together. Components of an operon include:
- Promoter: DNA site for RNA polymerase binding.
- Operator: "On-off switch," located within or near the promoter.
- Genes: Multiple related genes transcribed as one mRNA molecule.
The trp Operon: A Repressible Operon
- E. coli synthesizes tryptophan through a series of enzymes coded by the trp operon.
- Normally "on," but can be turned "off" by a repressor protein.
- trp Repressor:
- Encoded by the regulatory gene (trpR), separate from the operon.
- Inactive by default, activated by binding tryptophan (corepressor).
- Activated repressor binds to operator, blocking RNA polymerase and halting transcription.
- When tryptophan levels drop, repressor detaches, transcription resumes.
The lac Operon: An Inducible Operon
- Controls lactose metabolism in E. coli, normally "off," turned "on" by the presence of lactose.
- lac Repressor:
- Active by default, binds operator and prevents transcription.
- Allolactose (inducer) binds to the repressor, causing it to detach from the operator, allowing gene transcription.
- When lactose is present, enzymes to metabolize lactose (e.g., β-galactosidase) are synthesized rapidly.
Negative vs. Positive Gene Regulation
- Negative control: Operons are switched off by repressor proteins (both trp and lac operons are examples).
- Positive control: Regulatory proteins (activators) directly stimulate gene transcription.
- lac operon example of positive control:
- When glucose is low, cyclic AMP (cAMP) increases.
- cAMP binds CRP (cAMP receptor protein), activating it.
- Active CRP binds near lac promoter, increasing transcription rate significantly.
- When glucose levels are high, CRP is inactive, and transcription rate is low even if lactose is present.
- Thus, lac operon has dual regulation: negative (lac repressor) and positive (CRP-cAMP).
In a Nutshell
Bacteria regulate gene expression primarily at the transcriptional level using operons. The trp operon (repressible) and lac operon (inducible) illustrate how bacterial cells respond to their nutritional environment efficiently. Operons allow coordinated control, saving energy and resources by producing enzymes only when necessary.