10.Transcription control in prokaryotes. Lactose and tryptophan operons.
Covers how prokaryotic transcription is switched on and off, through positive and negative control of initiation. The lac operon is taken in turn: its regulatory regions, the three enzyme-coding genes, how lactose inducer and glucose levels with CAP–cAMP decide expression, and a summary of the four combinations. The tryptophan operon follows, showing repression by a corepressor and attenuation via leader-peptide ribosome stalling and stem-loop formation. Four pictures accompany the text.
Topic 10 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 10 sections, about 699 words (5 min of reading), with 25 flashcards.
In this note
Transcription control in prokaryotes. Lactose and tryptophan operons.
Control of transcription in prokaryotes – operons
What are operons?
Lac Operon
What makes the lac operon turn on?
Inducible Genes – Operon Model
Catabolic pathways.
Summary
Tryptophan Operon
Repressible Genes – Operon Model
Control of transcription in prokaryotes – operons
Control of transcription initiation can be:
Positive control – increases transcription when activators bind to DNA.
Negative control – reduces transcription when repressors bind to operators (DNA regulatory regions).
What are operons?
Operons = organization of genes involved in gene expression. Some are activated when needed and suppressed otherwise.
Lac Operon
The lac operon contains genes for the use of lactose as an energy source; these genes are transcribed by a single mRNA.
Regulatory regions of the lac operon include the CAP binding site, promoter, and the operator.
Coding region contains genes for 3 enzymes: β-galactosidase, permease, and transacetylase.
The promoter is the binding site for RNA polymerase, the enzyme that performs transcription.
The operator is a negative regulatory site bound by the lac repressor protein.
The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription.
The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP).
When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter.
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