Chapter 17 – STUDY GUIDE: Control of gene expression in bacteriaLearning Objectives: Using the lac operon in bacteria as a model, • Explain the role of repressors and inducers in negative control of gene expression. • Explain the role of CAP protein and cAMP in positive control of gene expression. Regulation of gene expression in bacteria is critical. Bacteria occupy a variety of habitats and often survive by switching among several different food sources. Each type of nutrient taken up by a cell requires specific membrane transport proteins and enzymes to metabolize it.Mechanisms of regulation: 1. Transcriptional control  If a gene is not transcribed, no mRNA is made. Transcriptional control isthe most energy-efficient control mechanism (slow but efficient). 2. Translational control (or post-transcription control)  mRNA is not translated, or the translation rate is controlled, due to: (1) Degradation of the mRNA by cellular ribonucleases (2) Lack of initiation by a ribosome on the mRNA (3) Efficiency of the interactions among tRNAs, elongation factors, and ribosomes Translational control is a rapid method of control that allows cells to quickly change which proteins are being produced. 3. Post-translational regulation  Proteins are manufactured in an inactive form  activated by chemical modification when needed; example: phosphorylationPost-translational regulation is the least efficient, but fastest control mechanism: Proteins are already made and can rapidly be activated.All levels of control of gene expression occur in bacteria. Genes are not simply on or off; the level of expression is highly variable. Variation in gene expression allows organisms to respond to their environment. a. Some genes, like those involved in glycolysis, are expressed constitutively (all of the time). b. Other genes are regulated, either induced or repressed. Metabolizing lactose, a model system: The lac operonE. coli can metabolize a wide variety of sugars, such as glucose, lactose, and others; however, glucoseis preferred. If lactose is available in the medium, it is transported into the cell. Then the cell produces the enzyme ß-galactosidase, which appears in the cytoplasm and breaks down lactose into glucose and galactose. Lactose act as an inducer of the lac operon structural genes involving ß-galactosidase gene. Several genes are involved in lactosemetabolism.• Lactose is a disaccharide containinggalactose linked to glucose. To be metabolized by E. coli, it is acted on by three proteins.• Lactose is transported into the cell by a carrier protein (enzyme) called galactoside permease (fromlacY) • Lactose is hydrolyzed to glucose and galactose by the enzyme β-galactosidase (from lac Z gene)• A third enzyme, Transacetylase (from lac A gene) is also required for lactose metabolism, although its role in the process is not yet clear- protective role?.• When no lactose is present, levels of all three enzymes are low. When glucose is low and lactose is high, transcription/ translation of all three enzymes occurs rapidly. If glucose levels rise again, or lactose levels drop, the enzyme levels remain very low.• Compounds (such as lactose) that stimulate synthesis of an enzyme are called inducers. The enzymes produced are inducible enzymes.• Enzymes made all the time at a constant rate are constitutive enzymes.1A single promoter can control the transcription of adjacent genes  ways Bacteria regulate many genes together• Structural genes are blueprints that specify the primary structures (amino acid sequence) of a proteinmolecule. In other words, structural genes are those that can be transcribed into mRNA  proteins• The three of these that are involved in lactose metabolism are adjacent to each other on the E. coli chromosome.• All are transcribed together when a single promoter binds RNA polymerase.• Therefore, their synthesis is coordinated.• Because there is just a single promoter, all genes are transcribed efficiently into a single mRNA.• When these enzymes are not needed, the mRNA synthesis must be shut down.Operons are units of transcription in prokaryotes• Prokaryotes shut down transcription by placing an obstacle between the promoter and its structural genes.• Just downstream from the promoter (P), between the promoter and structural genes, is a DNA site called the operator(O).• If a specific protein, the repressor, binds to the operator, it creates an obstacle, and RNA polymeraseis blocked from transcribing the structural genes  negative control• When the repressor is not attached to the operator, mRNA synthesis proceeds.• The whole unit of genes and their DNA controls is called an operon (P+O+ structural genes)Operator–repressor control induces transcription in the lac operon• The repressor protein has two binding sites: one for the operator and the other for inducers.• Binding of the repressor by the inducer molecules (e.g., an analog of lactose) changes the shape of the repressor by allosteric modification.The change in shape prevents the repressor from binding to the operator. • Thus, RNA polymerase can bind to the promoter and start gene transcription of the lac operon.• If the concentration of the inducer (lactose)drops, the repressor binds to the operator, andthe enzymes for lactose metabolism are notsynthesized.• If the concentration of lactose rises, therepressor binds with lactose  cannot bind tothe operator. The enzymes for lactose metabolism aresynthesized.• The repressor protein is coded for by theregulatory gene. The regulatory gene that codes for the lac repressor is the laci (inducibility) gene.• The lac i gene just happens to be located near the lac structural genes. However, not all regulatory genes are near the operons they control. Regulatory genes like laci have their own promoter.• The lac i gene is expressed constitutively, meaning expression is constant. Protein synthesis can be controlled by increasing promoter efficiency  Positive regulation:• Another way to regulate transcription is to make the promoter sequence of the operon work more efficiently.• When glucose is high, even when lactose is available, the lac operon  inefficient transcription. Why?  catabolite repression.• When glucose is low, and lactose is available, lac structural genes are transcribed more efficiently• Low glucose levels cause elevated levels of cyclic AMP (cAMP).2• The