Operons: The Basic Concept

1. 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

2. Replicationfork Origin of replication Termination of replication Figure 18.14 • Bacterial cells divide by binary fission • Which is preceded by replication of the bacterial chromosome

3. Mutation and Genetic Recombination as Sources of Genetic Variation • Since bacteria can reproduce rapidly • New mutations can quickly increase a population’s genetic diversity

4. An operon • Is usually turned “on” • Can be switched off by a protein called a repressor

5. trp operon Promoter Promoter Genes of operon RNA polymerase Start codon Stop codon 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 • The trp operon: regulated synthesis of repressible enzymes

6. 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

7. 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. 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 • The lac operon: regulated synthesis of inducible enzymes

9. 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

10. Inducible enzymes • Usually function in catabolic pathways • Repressible enzymes • Usually function in anabolic pathways

11. 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. Positive Gene Regulation • Some operons are also subject to positive control • Via a stimulatory activator protein, such as catabolite activator protein (CAP)

13. Operator RNA polymerase can bindand transcribe Promoter DNA lacl lacZ CAP-binding site ActiveCAP cAMP Inactive lac repressor InactiveCAP (a) 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. 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. Promoter Operator DNA lacl lacZ CAP-binding site RNA polymerase can’t bind InactiveCAP Inactive lac repressor 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. (b) Figure 18.23b • When glucose levels in an E. coli cell increase • CAP detaches from the lac operon, turning it off