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Gene Regulation in Bacteria

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Gene Regulation in Bacteria 11/15/12 E.coli bacteria eat whatever we eat! Gene regulation in bacteria Today’s Topics: But ALL organisms must • Transcriptional control adjust to changes in their – Cells adjust to their environment environment. by turning genes on and off • The operon concept – Operators, Promoters, Repressors, Activators • Repressible operons (e.g. trp) • Inducible operons (e.g. lac) 16 Nov 2012 Regulation of metabolism occurs at Types of Regulated Genes two levels: (a) Regulation of enzyme (b) Regulation of enzyme activity production – Adjusting the activity of Precursor • Constitutive genes are always expressed metabolic enzymes already Feedback • Tend to be vital for basic cell functions (often called inhibition Enzyme 1 present Gene 1 “housekeeping genes”) Enzyme 2 Gene 2 Regulation • How? of gene expression • Inducible genes are normally off, but can be turned – Regulating the genes Enzyme 3 Gene 3 – on when substrate is present encoding the metabolic Gene 4 Enzyme 4 • Common for catabolic enzymes (i.e. for the utilization of enzymes – particular resources) Enzyme 5 Gene 5 • How? Tryptophan • Repressible genes are normally on, but can be turned off when the end product is abundant • Common for anabolic enzymes In bacteria, related genes are often clustered into The trp Operon Operons Controlled by a single P R P O 1 2 3 promoter and operator Operons have: 5 genes: E, D, C, B, A 1. Several genes for metabolic enzymes 2. One promoter 3. An operator, or control site (“on-off” switch) 4. A separate gene that makes a repressor or activator protein that binds to the operator Same order as enzymes for trp synthesis 1 11/15/12 The trp operon: regulated synthesis Terminology of repressible enzymes Regulatory trp operon • Promoters and Operators are DNA sequences gene Promoter upstream of genes Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator • Repressors and Activators are proteins that bind to RNA mRNA 3! polymerase DNA and control transcription. mRNA 5! 5! E D C B A • Co-repressors and Inducers are small “effector” Protein Polypeptides that make up molecules that bind to repressors or activators enzymes for tryptophan synthesis Figure 18.3 Tryptophan absent ! repressor inactive ! operon “on” Active repressor can bind to operator and Tryptophan changes the shape of the block transcription repressor protein so it can bind DNA DNA No RNA made mRNA Protein Active Tryptophan repressor (corepressor) Tryptophan present ! repressor active ! operon “off” Figure 18.3 • The lac operon: regulated synthesis of The lac operon inducible enzymes Promoter Regulatory Operator gene • Lacose DNA lacl lacZ No RNA made 3! RNA mRNA polymerase 5! Active Protein repressor (a) 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. Figure 18.4 2 11/15/12 Positive vs Negative Gene lac operon Regulation DNA lacl lacz lacY lacA • Both the trp and lac operons involve negative control RNA polymerase of genes 3! – because the operons are switched off by the active form of mRNA mRNA 5'5! 5! the repressor protein -Galactosidase Permease Transacetylase Protein " • Some operons are also subject to positive control – An activator protein is required to start transcription. Allolactose Inactive – E.g. catabolite activator protein (CAP) (inducer) repressor (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 18.4 Positive Gene Regulation: CAP When glucose is abundant, lac is not transcribed Glucose is the preferred food source so the lac operon is CAP detaches from the promoter activated only when glucose is scarce. Promoter DNA lacl lacZ Without CAP, Operator Promoter RNA polymerase Active CAP DNA can’t bind lacl lacZ helps RNA Inactive CAP-binding site RNA Operator Inactive lac polymerase polymerase CAP can bind repressor cAMP and transcribe bind to Active CAP promoter Inactive Inactive lac CAP repressor “catabolite activator protein” Figure 18.5 Figure 18.5 3 .
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