Gene Expression of Prokaryotes

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University of Massachusetts Amherst
Mark Huyler

Gene Expression in Prokaryotes  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 Introduction  A cell does not express all of its genes at the same time. Instead, the are very selective about the genes … Gene Regulation and Information Flow  Eschericha 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 uses of sugars  Gene expression in bacteria was predicted to be triggered by specific signals from the environment Mechanisms of Regulation …an overview… Transcriptional Control of Gene Expression  Transcriptional control occurs when the cell does not produce mRNA for specific enzymes o The cell avoids the production of these enzymes by utilizing regulatory proteins that prevent RNA polymerase from binding to a promoter o DNA  RNA  protein activated protein  Translational control allows the cell to prevent the translation of an mRNA molecule that has already been transcribed. This can occur through many mechanisms: o Regulatory molecules can speed up o …. 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—x-->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 chance which proteins are produced  Post-translational control provides the most rapid response but it is energetically expensive Mechanisms of Regulation…. 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 E. coli must transport it into the cell, where the enzyme B-galactosidase can cleave it to produce glucose and galactose  E. coli produces high levels of B-galactosidae only when lactose is present in the environment  Thus, lactose acts as an inducer – a molecule that stimulates the expression of a specific gene Identifying Genes under Regulatory Control  To find the genes that code for B-galactosidase and the membrane transport protein that brings lactose into the cell. Monod and Francois Jacob isolated and analyzed E. coli mutants that could not metabolize lactose Replicator Plating to Find Mutant Genes  Indicator plates can also be used o These allow researchers to directly observe mutants with metabolic deficiencies o These plates made it possible for the to identify colonies without functioning B-galatosidase Different Classes of Lactose Metabolism Mutants  The three genes involved in lactose metabolism were named LacZ, LazY and LacI  Three classes of E. coli mutants defective in lactose, metabolism were identified: o LacZ mutants lack functional B-galactosidase o LacY mutants lack the membrane protein galactoside permease and so cannot transport lactose into the cell o LacI mutants are called constitutive mutants because they produce g-galactosidase and galactoside permease even when lactose is absent Several Genes are Involved in Metabolizing Lactose  The LacZ and LacY code for proteins involved in lactose metabolism, while the LacI gene product serves as a regulatory function  I 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 LacZ, LacY and LacI genes are… Mechanisms of Negative Control: The Repressor  Transcription can be regulated via negative control or positive control  Negative control occurs when regulatory proteins bind to DNA and sh
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