1. Operons

2. Structural gene Operon Polycistronic mRNA Operator Regulator gene Repressor Overview animation Overview animation

3. PROMOTER---OPERATOR----gene 1—gene 2—gene 3 “Regulator gene” (located elsewhere on chromosome)

5. Normally OFF, induced (turned on) only when needed Ex. Lac operon (3 enzymes for catabolism of lactose) repressor binds at operator : no transcription lactose binds repressor, comes OFF operator : Transcription can proceed!

6. This is the graphic from your textbook. I like the one on the next slide better

8. Inducible operon animation Inducible operon animation (lac operon)

9. Normally ON, repressed (turned off) when not needed Example: Genes for amino acid biosynthetic enzymes Arginine operon and tryptophan operon are examples. Operon is actively transcribing Arginine (or tryptophan) is synthesized If arginine (or tryptophan) accumulates beyond useful levels, arg (trp) acts as COREPRESSOR Corepressor + repressor protein bind to operator, SHUT OFF OPERON

10. This is the graphic from your textbook. I like the one on the next slide better

13. Repressible operon animation Repressible operon animation (trp operon)

15. Activator protein instead of a repressor. The activator protein promotes the binding of RNA Polymerase to the promoter and thereby acts to increase mRNA synthesis. Maltose operon is an example

16. Maltose operon

17. Catabolite repression