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Lecture 4

BIL 250 Lecture 4: ch16

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BIL 250

Prokaryotes Regulate Gene Expression in Response to Environmental Conditions (internal and external) Gene regulation has been studied extensively in E.coli Highly efficient genetic mechanisms have evolved to turn transcription of specific genes on and off depending on a cells metabolic need for specific gene products Gene expression regulation in prokaryotic occurs mostly at the level of regulation of transcription These responses can be due to changes in the environment as well as nonenvironmentally regulated cellular activity and cell division Regulation of Gene Expression in Prokaryotes Transcription in genes Genetic regulatory proteins bind to the DNA and control the rate of transcription In attenuation, transcription terminates soon after it has begun due to the formation of a transcriptional terminator Bacteria adapt to their environment by producing certain enzymes (inducible enzymes) only when specific substrates (inducers) are present Enzymes continuously produced regardless of the chemical makeup of the environment are called constitutive enzymes An abundance of an end product in the environment represses gene expression Repressible system Regulation of the inducible or repressible type may be under positive control or negative control Negative control: genetic expression occurs unless it is shut off by some form of a regulator molecule Positive Control: transcription or genetic expression occurs only if a regulator molecule directly stimulates RNA production Lactose Metabolism in E.Coli is Regulated by an Inducible System In the presence of lactose, concenrations of the enzymes responsible for lactose metabolism increase rapidly from a few molecules to thousands per cell The enzymes responsible for lactose metabolism are inducible, and lactose is the inducer Lactose to glucose (disaccharide to monosaccharide) Prokaryotic genes that code for enzymes with related funcitons are organized in clusters on the bacterial chromosome: operons Transcription is under the control of a single transcription regulatory region (polycistronic) Regulatory regions are almost always located on the same chromosmes, upstream of the gene cluster they control
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