1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expression E. coli, a type of bacteria that lives in the human colon – Can tune its metabolism to the changing environment and food sources

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings This metabolic control occurs on two levels – Adjusting the activity of metabolic enzymes already present – Regulating the genes encoding the metabolic enzymes Figure 18.20a, b (a) Regulation of enzyme activity Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 Regulation of gene expression Feedback inhibition Tryptophan Precursor (b) Regulation of enzyme production Gene 2 Gene 1 Gene 3 Gene 4 Gene 5 – –

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Operons: The Basic Concept In bacteria, genes are often clustered into operons, composed of – An operator, an “on-off” switch – A promoter – Genes for metabolic enzymes

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An operon – Is usually turned “on” – Can be switched off by a protein called a repressor

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The trp operon: regulated synthesis of repressible enzymes Figure 18.21a (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Genes of operon Inactive repressor Protein Operator Polypeptides that make up enzymes for tryptophan synthesis Promoter Regulatory gene RNA polymerase Start codon Stop codon Promoter trp operon 5 3 mRNA 5 trpD trpE trpCtrpB trpA trpR DNA mRNA ED C BA

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA mRNA Protein Tryptophan (corepressor) Active repressor No RNA made Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. (b) Figure 18.21b

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 In an inducible operon – Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The lac operon: regulated synthesis of inducible enzymes Figure 18.22a DNA mRNA Protein Active repressor RNA polymerase No RNA made lacZ lacl Regulatory gene Operator Promoter Lactose absent, repressor active, operon off. The lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. (a) 5 3

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings mRNA 5' DNA mRNA Protein Allolactose (inducer) Inactive repressor lacl lacz lacYlacA RNA polymerase PermeaseTransacetylase  -Galactosidase 5 3 (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. mRNA 5 lac operon Figure 18.22b

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inducible enzymes – Usually function in catabolic pathways Repressible enzymes – Usually function in anabolic pathways

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Regulation of both the trp and lac operons – Involves the negative control of genes, because the operons are switched off by the active form of the repressor protein

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Positive Gene Regulation Some operons are also subject to positive control – Via a stimulatory activator protein, such as catabolite activator protein (CAP)

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Promoter Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized. If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway. (a) CAP-binding site Operator RNA polymerase can bind and transcribe Inactive CAP Active CAP cAMP DNA Inactive lac repressor lacl lacZ Figure 18.23a In E. coli, when glucose, a preferred food source, is scarce – The lac operon is activated by the binding of a regulatory protein, catabolite activator protein (CAP)

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings When glucose levels in an E. coli cell increase – CAP detaches from the lac operon, turning it off Figure 18.23b (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription. Inactive lac repressor Inactive CAP DNA RNA polymerase can’t bind Operator lacl lacZ CAP-binding site Promoter