11/25/12

The trp : Gene regulation, continued Repressible are normally on

Regulatory trp operon Today’s topics: RNA Pol • Review operons gene 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 synthesis • Gene regulation in Figure 18.3 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 to operator and block Inducible operons are normally off

DNA

No RNA made

When lactose is present, mRNA repressor can no longer bind DNA. Transcription occurs

Protein Active ()

DNA binding proteins can be either 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 () repressor when lactose is present? absent? What will happen if there is a deletion of the: • operator? • promoter? CAP helps RNA polymerase • 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 • • Short- and long-term regulation

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