Introduction - Control of Gene Expression in BacteriaSlide 2Gene RegulationMechanisms of Regulation―An OverviewTranscriptional Control of Gene ExpressionTranslational Control of Gene ExpressionPost-Translational Control of Gene ExpressionControl of Gene Expression in BacteriaSlide 9Slide 10Metabolizing Lactose―A Model SystemSlide 12Slide 13Slide 14OPERONS:The lac OperonSeveral Genes Are Involved in Metabolizing LactoseSlide 18Slide 19Lac operon of E.coli :Mechanisms of Negative Control: The RepressorSlide 22Slide 23Mechanisms of Positive Control: Catabolite RepressionSlide 25The CAP Protein and Binding SiteSlide 27Glucose Influences Formation of the CAP-cAMP ComplexSlide 29Slide 30Slide 31Slide 32Why Has the lac Operon Model Been So Important?Slide 34Different Classes of Lactose Metabolism MutantsSlide 36Slide 37Tryptophan operonSlide 3917. Key Concepts Control of Gene Expression in Bacteria© 2011 Pearson Education, Inc.Introduction - Control of Gene Expression in Bacteria•A cell does not express all of its genes all of the time. Instead, they are very selective about the genes they express, how strongly they are expressed, and when they are expressed.•Gene expression occurs when a gene product is actively being synthesized and used in a cell. Regulation of gene expression is critical to the efficient use of resources and thus survival.© 2011 Pearson Education, Inc.A typical Bacterial genome  ~4300 genesHuman genome  ~ 100,000 genes•Only a fraction of these genes are expressed at any given time! •Proteins made at specific times during cell cycle are Inducible. Proteins made all the time at a constant rate are constitutive.•Requirements of some gene products change over time. Example: need for enzymes in metabolism – depends on the food availability. •Regulation of gene expression is essential in making optimal use of available energy.Pgs 296-300© 2011 Pearson Education, Inc.Gene Regulation•Unlike plant and animal cells, most bacteria are exposed to a constantly changing physical and chemical environment. •Within limits, bacteria can react to changes in their environment through changes in synthesis /modification of of structural proteins, transport proteins, toxins, enzymes, etc., which adapt them to a particular situation.•Generally, bacteria do not synthesize degradative (catabolic) enzymes unless the substrates for these enzymes are present in their environment.•Control mechanisms?© 2011 Pearson Education, Inc.Mechanisms of Regulation―An Overview•Information flow occurs in three steps, represented by arrows:DNA mRNA protein activated protein•Genes can be under transcriptional, translational, or post-translational control.–All three types of regulation occur in bacteria.© 2011 Pearson Education, Inc.Transcriptional Control of Gene Expression•Transcriptional control occurs when the cell does not produce mRNA for specific enzymes.–The cell avoids the production of these enzymes by utilizing regulatory proteins that prevent RNA polymerase from binding to a promoter.DNA mRNA protein activated proteinx© 2011 Pearson Education, Inc.Translational Control of Gene Expression•Translational control allows the cell to prevent the translation of an mRNA molecule that has already been transcribed. This can occur through many mechanisms:–Regulatory molecules can speed up mRNA degradation.–Translation initiation can be altered.–Translation proteins can be affected.DNA mRNA protein activated proteinx© 2011 Pearson Education, Inc.Post-Translational Control of Gene Expression•Post-translational control occurs when the cell fails to activate a manufactured protein by chemical modification. DNA mRNA protein activated proteinx© 2011 Pearson Education, Inc.Control of Gene Expression in BacteriaAll three forms of gene expression control occur in bacteria.•Transcriptional control is slow but efficient. •Translational control allows a cell to quickly change which proteins are produced. •Post-translational control provides the most rapid response but is energetically expensive.© 2011 Pearson Education, Inc.Escherichia coli bacteria has served as an excellent model organism for the study of prokaryotic gene regulation.Like most bacteria, E. coli can use a wide array of carbohydrates to supply carbon and energy. Control of gene expression allows E. coli to respond to its environment and switch its use of sugars.Gene expression in bacteria was predicted to be triggered by specific signals from the environment.Bacteria in the human gut?© 2011 Pearson Education, Inc.E. coli cells live in our intestines and use glucose as their preferred energy source. When a person drinks milk, these cells encounter an alternative energy source—the sugar lactose. Although E. coli can metabolize lactose, it will make the proteins for lactose metabolism only when two conditions are met: glucose levels are low and lactose is present. This adaptation saves energy and confers a selective advantage in that only necessary proteins are produced at any given time.© 2011 Pearson Education, Inc.Metabolizing Lactose―A Model System•E. coli’s preferred carbon source is glucose, and uses lactose only when glucose is depleted.•Before it can utilize lactose, E. coli must transport it into the cell, where the enzyme -galactosidase can cleave it to produce glucose and galactose. •E. coli produces high levels of -galactosidase (enzyme that digest Lactose) only when lactose is present in the environment. •Thus, lactose acts as an inducer—a molecule that stimulates the expression of a specific gene.© 2011 Pearson Education, Inc. Lactose disaccharide = (galactose – glucose)  milk sugar (14) glucosidic linkageFrom Chapter 5:© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.OPERONS: • In Prokaryotes, genes that code for enzymes in certain metabolic pathways are often controlled as a group; these under the control of a common promoter. Such a group of genes  called an “operon”Structural genes codes for specific protein sequence© 2011 Pearson Education, Inc.The lac Operon•Jacob and Monod coined the term operon for a set of coordinately regulated bacterial genes that are transcribed together into one mRNA. –The group of genes involved in lactose metabolism was thus termed the lac operon.•Later a fourth gene, called lacA, was discovered to be part of the lac operon.