Control of Gene Expression

2. Control of Gene Expression

3. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • Gene regulation is the turning on and off of genes. • Gene expression is the overall process of information flow from genes to proteins. • The control of gene expression allows cells to produce specific kinds of proteins when and where they are needed. • Our earliest understanding of gene control came from the study of E. coli.

4. Figure 11.1a

5. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • An operon is a cluster of genes with related functions, along with the control sequences. • With rare exceptions, operons only exist in prokaryotes.

6. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • When an E. coli encounters lactose, all the enzymes needed for its metabolism are made at once using the lactose operon. • The lactose (lac) operon includes • three adjacent lactose-utilization genes, • a promoter sequence, a site where RNA polymerase binds and initiates transcription of all three lactose genes, and • an operator sequence, where a repressor can bind and block RNA polymerase action.

7. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • E. coli uses three enzymes to take up and start metabolizing lactose only when lactose is present. • The genes coding for these three enzymes are • located next to each other and • regulated as a single unit.

8. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • Regulation of the lac operon • A regulatory gene, located outside the operon, continually codes for a repressor protein. • In the absence of lactose, the repressor binds to the operator and prevents RNA polymerase action. • Lactose inactivates the repressor, so • the operator is unblocked, • RNA polymerase can bind to the promoter, and • all three genes of the operon are transcribed.

9. Operon turned off (lactose is absent): OPERON Regulatorygene Promoter Operator Lactose-utilization genes DNA RNA polymerase cannotattach to the promoter mRNA Figure 11.1b-0 Activerepressor Protein Operon turned on (lactose inactivates the repressor): DNA RNA polymerase isbound to the promoter mRNA Translation Protein Inactiverepressor Lactose Enzymes for lactose utilization

10. Operon turned off (lactose is absent): OPERON Figure 11.1b-1 Regulatorygene Promoter Operator Lactose-utilization genes DNA RNA polymerase cannotattach to the promoter mRNA Activerepressor Protein

11. Operon turned on (lactose inactivates the repressor): Figure 11.1b-2 DNA RNA polymerase isbound to the promoter mRNA Translation Protein Lactose Enzymes for lactose utilization Inactiverepressor

12. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • There are two types of repressor-controlled operons. • The lac operon is an example of an inducible operon that is usually turned off but can be stimulated (induced) by a molecule—in this case, by lactose. • The trp operon is an example of a repressible operon that is normally turned on but can be inhibited (repressed) when a specific molecule (such as amino acid tryptophan) is present in abundance.

13. lac operon (inducible) trp operon (repressible) Operator Gene Promoter Figure 11.1c DNA Active repressor Activerepressor Tryptophan Inactiverepressor Inactiverepressor Lactose

14. 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • Another type of operon control involves activators, proteins that turn operons on by • binding to DNA and • stimulating gene transcription. • Activators help control a wide variety of operons.