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Control of Gene Expression Chapter 16

Control of Gene Expression Controlling gene expression is often accomplished by controlling transcription initiation Regulatory proteins bind to DNA May block or stimulate transcription Prokaryotic organisms regulate gene expression in response to their environment Eukaryotic cells regulate gene expression to maintain homeostasis in the organism

Prokaryotic regulation Control of transcription initiation Positive control – increases frequency of initiation of transcription Activators enhance binding of RNA polymerase to promoter Effector molecules can enhance or decrease Negative control – decreases frequency Repressors bind to operators in DNA Allosterically regulated Respond to effector molecules – enhance or abolish binding to DNA

Prokaryotic cells often respond to their environment by changes in gene expression Genes involved in the same metabolic pathway are organized in operons Induction – enzymes for a certain pathway are produced in response to a substrate Repression – capable of making an enzyme but does not

lac operon Contains genes for the use of lactose as an energy source b-galactosidase (lacZ), permease (lacY), and transacetylase (lacA) Gene for the lac repressor (lacI) is linked to the rest of the lac operon

The lac operon is negatively regulated by a repressor protein lac repressor binds to the operator to block transcription In the presence of lactose, an inducer molecule (allolactose) binds to the repressor protein Repressor can no longer bind to operator Transcription proceeds Even in the absence of lactose, the lac operon is expressed at a very low level

a. Glucose Low , Inducer Present, Promoter Activated DNA Allolactose Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glucose Low , Inducer Present, Promoter Activated DNA Allolactose Repressor will not bind to DNA cAMP–CAP binds to DNA CAP- binding site mRNA Glucose level is low cAMP is high A CAP Y cAMP cAMP Z cAMP activates CAP by causing a conformation change RN A polymerase is not blocked and transcription can occur a.

blocked by the lac repressor Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glucose High, Inducer Absent, Promoter Not Activated Glucose is available cAMP level is low Repressor binds to DNA A CAP does not bind Y Effector site is empty, and there is no conformation change RNA polymerase is blocked by the lac repressor b.

Glucose repression Preferential use of glucose in the presence of other sugars Mechanism involves activator protein that stimulates transcription Catabolic activator protein (CAP) is an allosteric protein with cAMP as effector Level of cAMP in cells is reduced in the presence of glucose so that no stimulation of transcription from CAP-responsive operons takes place Inducer exclusion – presence of glucose inhibits the transport of lactose into the cell

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trp operon Genes for the biosynthesis of tryptophan The operon is not expressed when the cell contains sufficient amounts of tryptophan The operon is expressed when levels of tryptophan are low

The trp operon is negatively regulated by the trp repressor protein trp repressor binds to the operator to block transcription Binding of repressor to the operator requires a corepressor which is tryptophan Low levels of tryptophan prevent the repressor from binding to the operator

Eukaryotic Regulation Control of transcription more complex Major differences from prokaryotes Eukaryotes have DNA organized into chromatin Complicates protein-DNA interaction Eukaryotic transcription occurs in nucleus Amount of DNA involved in regulating eukaryotic genes much larger

Transcription factors General transcription factors Necessary for the assembly of a transcription apparatus and recruitment of RNA polymerase II to a promoter TFIID recognizes TATA box sequences Specific transcription factors Increase the level of transcription in certain cell types or in response to signals

Promoters form the binding sites for general transcription factors Mediate the binding of RNA polymerase II to the promoter Enhancers are the binding site of the specific transcription factors DNA bends to form loop to position enhancer closer to promoter

Eukaryotic chromatin structure Structure is directly related to the control of gene expression DNA wound around histone proteins to form nucleosomes Nucleosomes may block access to promoter Histones can be modified to result in greater condensation

Methylation once thought to play a major role in gene regulation Many inactive mammalian genes are methylated Lesser role in blocking accidental transcription of genes turned off Histones can be modified Correlated with active versus inactive regions of chromatin Can be methylated – found in inactive regions Can be acetylated – found in active regions