Key Concepts In bacteria gene expression can be controlled at three levels transcription translation or post translation protein activation Changes in gene expression allow bacterial cells to respond to environmental changes Transcriptional control can be negative or positive Negative control occurs when a regulatory protein prevents transcription Positive control occurs when a regulatory protein increases the transcription rate 2011 Pearson Education Inc Introduction 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 Gene Regulation and Information Flow Escherichia coli 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 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 posttranslational 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 x mRNA protein 2011 Pearson Education Inc activated protein Control of Gene Expression in Bacteria All 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 2011 Pearson Education Inc Mechanisms of Regulation An Overview The level of expression of different genes can be highly variable Variation in gene expression allows cells to respond to changes in their environment 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 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 2011 Pearson Education Inc Several Genes Are Involved in Metabolizing Lactose The lacZ and lacY genes code for proteins involved in lactose metabolism while the lacI gene product serves a regulatory function In the absence of lactose the lacI gene product shuts down expression of lacZ and lacY When lactose is present however transcription of lacZ and lacY is induced Further studies revealed that the lacZ lacY and lacI genes are located close together on the circular E coli chromosome This suggested that lacI could control both lacZ and lacY 2011 Pearson Education Inc 2011 Pearson Education Inc Mechanisms of Negative Control The Repressor Transcription can be regulated via negative control or positive control Negative control occurs when a regulatory protein binds to DNA and shuts down transcription Positive control occurs when a regulatory protein binds to DNA and triggers transcription 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc Mechanisms of Negative Control The Repressor Szilard and Monod hypothesized that the lacI gene codes for a repressor that exerts negative control over the lacZ and lacY genes They hypothesized that the repressor would bind directly to the DNA on or near the promoter for the lacZ and lacY genes Lactose then would interact with the repressor in a way that makes the repressor release from its binding site Lactose thus would induce transcription by removing negative control 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc Why Has the lac Operon Model Been So Important Regulation of the lac operon provided an important model system in genetics We now know that gene expression of many bacterial operons is regulated by physical contact between regulatory proteins and specific regulatory sites on DNA In addition as in the lac operon the activity of many other key regulatory proteins is regulated by post translational control 2011 Pearson Education Inc Mechanisms of Positive Control Catabolite Repression Transcription of the lac operon is greatly reduced when glucose is present even when lactose is also available When glucose is already available the cell does not need to produce more by cleaving lactose This is an example of catabolite repression Occurs when one of the product molecules the catabolite of a reaction represses the production of the enzyme s responsible for that reaction In the case of the lac operon glucose is the catabolite 2011 Pearson Education Inc 2011 Pearson Education Inc