Operons fancy light switches

1. Operonsfancy light switches Ch 18

2. We all seek control • Think of a light switch • Two purposes, on and off • Sometimes we need light • Advantages: have it when needed, i.e. the expression of what we want • We can use this built in convenience mechanism to use things when necessary • Sometimes we don’t need light • Advantages: conservation of energy, i.e. the control of or limiting of what we may need but not at the current moment (ability to go to sleep)

3. Operons work similarly They can be turned on • Inducible Or they can be turned off • Repressible Some also work in response to differing amounts of product in the environment

4. Operon terminology Relate to gene structure • RNA polymerase: • Promoter: • Operator: • Repressor: • Inducer: • Structural genes: • Activator protein:

5. Repressible and Inducible Operons: Two Types of Negative Gene Regulation • In a repressible operon • Binding of a specific repressor protein to the operator shuts off transcription • Usually function in catabolic pathways • In an inducible operon • Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription • Usually function in anabolic pathways

6. Think like a bacterium • Bacteria can manufacture trp, we cannot thus it’s an essential amino acid for humans • For bacteria though, when tryptophan absent... • Bacteria want to make trp • They need to express the genes necessary to make trp • Any trp repressors would be inactive • The operon would be “on”

7. trp operon Promoter Promoter Genes of operon Start codon Stop codon RNA polymerase trpR trpD trpC trpB trpE trpA DNA Operator Regulatory gene 3 mRNA 5 mRNA 5 C E D B A Polypeptides that make up enzymes for tryptophan synthesis Inactiverepressor Protein (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Figure 18.21a Repressible: trp operon

8. So what happens when... • Trp is abundant? • Again, think like a bacterium. • If you have a lot of something around... • Why make more of it? • Shut off the light switch! • Trp itself binds to the repressor to inactivate the biosynthesis of trp • Energy conservation!!! OPERON OFF

9. DNA No RNA made mRNA Active repressor Protein Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. Figure 18.21b

10. Inducible operons • Lactose is a sugar obtained from diet • Again, imagine you’re a bacterium • Lactose is present or floating around and you want to use it. What to do? • IT’S ENERGY, USE IT! YOU’RE OPPORTUNISTIC AND MAY NEVER SEE THIS MUCH ENERGY EVER AGAIN! • HOW TO USE IT? Transcribe the genes that breakdown lactose! • Induce the operon!

11. lac operon DNA lacl lacz lacY lacA RNApolymerase 3 mRNA 5 mRNA 5' mRNA 5 -Galactosidase Permease Transacetylase Protein Inactiverepressor Allolactose(inducer) (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. Figure 18.22b

12. Lactose absent • No lactose around • What to do? • Shut off the operon! • Repress the expression of the genes that break down lactose • No need to waste energy making them!

13. Promoter Regulatorygene Operator DNA lacl lacZ NoRNAmade 3 RNApolymerase mRNA 5 Activerepressor Protein (a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator. Figure 18.22a

14. Remember... • Inducible enzymes • Usually function in catabolic pathways • Repressible enzymes • Usually function in anabolic pathways • http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html