PROKARYOTIC GENE REGULATION

• Transcription regulation is common mechanism in prokaryotes

• Negative Regulation = repressor binds to regulatory site to block transcription of active gene

• Positive Regulation = Activator binds to regulatory site to stimulate transcription; gene is off in absence of activator

TRANSCRIPTIONAL CONTROL OF GENE EXPRESSION INPROKARYOTES

The two well studied main mechanisms of transcriptional control of gene expression are:1. The operons : genes involved in a metabolic pathway are regrouped into a gene cluster controlled by common regulatory sequences and proteins. The expression of these genes are then rapid and synchronized. The operon model was developed by François Jacob and Jacques Monod (1961).

2. The cascades of gene expression : Under some environmental conditions, expression of a first set of genes can be “switch on”, and one or more of the products of this first gene set will “switch on” a second gene set. This event could be repeated many times to mobilize wider gene sets to achieve a special metabolic pathway.

• The operon consists of several structural genes required for lactose metabolism under the control of a single regulatory domain = coordinate regulation

• Repressor binding site = operator

• Site for transcriptional activation = promoter

• Inducible operon is activated by small molecule inducers; mode of regulation in degradative (catabolic) pathways

• Repressible operon is shut off by small molecule co-repressors; mode of regulation in biosynthetic (anabolic) pathways

Negative regulation: repressor protein blocks transcription:

• Inducible: inducer antagonizes repressor to allow transcription initiation

• Repressible: aporepessor combines with co-repressor to form functional repressor

• Positive Regulation: transcription occurs only if promoter is activated by transcriptional activator

• Negative regulation is more common in prokaryotes

• Positive regulation is more common in eukaryotes

• Autoregulation: protein regulates its own transcription

• Lactose degradation is regulated by the lactose (lac) operon

• The first regulatory mutations discovered affect lactose metabolism

• Structural proteins of the lac operon: B- galactosidase required for lactose metabolism and lactose permease required for lactose entry

• Lac operon gene expression can be inducible or constitutive

• The repressor is expressed constitutively (continuously) from the i gene and binds to the operator to block transcription

• The operon is inducible since lactose binds and inactivates the repressor to permit transcription initiation

• Lactose operator is essential site for repression

• Operator mutations are cis-dominant because only genes on the same genetic unit are affected

• Lactose promoter is essential site for transcription

• Lac operon contains linked structural genes and regulatory sites

• Lactose operon is also subject to positive regulation

• Positive regulation of the lac operon involves cAMP-CRP ( cyclic AMP receptor protein) which binds to the promoter to activate transcription by RNA polymerase

• cAMP-CRP complex regulates the activity of the lac operon

• The trp operon contains the structural genes which encode enzymes required for the synthesis of the amino acid tryptophan

• The trp operon is transcriptionally active unless tryptophan is present

• The trp operon is an example of a repressible system regulated by a negative feedback loop

• The trp operon is shut off when tryptophan binds to inactive aporepressor

• Tryptophan-repressor complex binds to operator to block transcription when tryptophan levels are high

• If tryptophan levels fall trp-repressor complex dissociates from operator

• Attenuation is a very sensitive form of translational regulation of the trp operon

• The trp attenuator sequence consists of 5’ base sequence in mRNA which is complementary and can base pair to form a stem and loop structure

• Attenuation results in the premature termination of mRNA synthesis due to stem and loop formation in the 5’ region of mRNA

• If tRNA-trp is present , synthesis of the leader peptide results in pairing of mRNA which blocks the action of RNA polymerase

• At low concentrations of tRNA-trp, the ribosome stalls, and the mRNA opens so that transcription continues

• Attenuation permits the cell to respond to tryptophan levels by expressing the genes needed for its synthesis when needed