11/25/12
The trp operon: Gene regulation, continued Repressible operons are normally on
Regulatory trp operon Today’s topics: RNA Pol • Review operons gene Promoter Repressor Genes of operon – Repressible operons (e.g. trp) DNA trpR trpE trpD trpC trpB trpA – Inducible operons (e.g. lac) Operator RNA mRNA • Positive regulation of lac 3! polymerase mRNA 5! (CAP) 5! • Practice applying the operon E D C B A Protein One long mRNA codes several concept polypeptides,Polypeptides each thatwith make its ownup start and enzymes for tryptophan synthesis stop codon • Gene regulation in Figure 18.3 prokaryotes vs eukaryotes 26 Nov 2012 The “operator” is a Repressor protein is particular sequence of inactive without bases where the repressor tryptophan. can bind.
Trp operon Tryptophan present: Active repressor can bind Lac operon to operator and block Inducible operons are normally off transcription
DNA
No RNA made
When lactose is present, mRNA repressor can no longer bind DNA. Transcription occurs
Protein Active Tryptophan repressor (corepressor)
DNA binding proteins can be either repressors Positive Gene Regulation- CAP
or activators, depending on how they intereact Promoter with RNA polymerase lacl DNA lacZ
Active form of Operator Figure 18.23a CAP helps RNA Active Activator cAMP CAP This configuration helps RNA polymerase bind polymerase bind to promoter, so Inactive lac Inactive repressor transcription can CAP start cAMP is the ligand that activates CAP
Repressor
This configuration blocks RNA polymerase
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When glucose is abundant, cAMP is Dual control of the lac operon used up Glucose must be absent • How does that affect transcription? Lactose must be present
DNA lacl lacZ
RNA polymerase + glucose off, because CAP not bound can’t bind + lactose
Inactive + glucose off, because repressor active Inactive lac CAP repressor - lactose and CAP not bound Figure 18.23b - glucose off, because repressor active - lactose
- glucose Operon active + lactose
The Lac operon CAP is a general control switch for alternative DNA lacl lacz lacY lacA RNA polymerase metabolism pathways 3! mRNA mRNA 5'5! 5!
-Galactosidase Permease Transacetylase Protein " Many Different Operons use CAP lac, gal, mal, ara, etc. Allolactose Inactive (inducer) repressor when lactose is present? absent? What will happen if there is a deletion of the: • operator? • promoter? CAP helps RNA polymerase • lac repressor gene? bind to promoter
Gene Regulation in Prokaryotes and Eukarykotes
• Prokaryotes • Eukaryotes – Operons – No operons • 27% of E. coli genes • but they still need to • (Housekeeping genes not coordinate regulation in operons) – More levels of control – simultaneous • RNA processing transcription and • Chromatin remodeling translation • Short- and long-term regulation
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