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Operons: The Basic Concept
Structural gene = Gene that codes for a polypeptide.
Operon = A regulated cluster of adjacent structural genes with related functions.
! Has a single promoter region, so an RNA polymerase will transcribe all
structural genes on an all-or-none basis.
! Transcription produces a single polycistronic mRNA with coding sequences
for all enzymes in a metabolic pathway.
Polycistronic mRNA = A large mRNA molecule that is a transcript of several genes.
! Contains stop and start codons for the translation of each polypeptide.
Operator = A DNA segment between an operon's promoter and structural genes, which controls access of RNA polymerase to structural genes.
! Acts as an on/off switch for movement of RNA polymerase and transcription
of the operon's structural genes.
Repressor = Specific protein that binds to an operator and blocks transcription of the operon.
! Blocks attachment of RNA polymerase to the promoter.
! Repressors are encoded by regulatory genes.
Regulatory genes = Genes that code for repressor or regulators of other genes.
! Are often located some distance away from the operons they control.
! Are involved in switching on or off the transcription of structural genes.
Corepressor = A molecule, usually a metabolite, that binds to a repressor protein, causing the repressor to change into its active conformation.
! Only the repressor-corepressor complex can attach to the operator and turn
off the operon.
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Repressible Versus Inducible Enzymes: Two Types of Negative Control
Repressible enzymes = Enzymes which have their synthesis inhibited by a metabolite (e.g., tryptophan).
Inducible enzymes = Enzymes which have their synthesis stimulated or induced by specific metabolites (i.e., lac operon).
Repressible Enzymes Inducible Enzymes
! Their genes are switched on until a
specific metabolite activates the repressor.
! Generally function in anabolic
pathways.
! Pathway end product switches off its
own production by repressing enzyme synthesis.
! Their genes are switched off until a
specific metabolite inactivates the repressor.
! Function in catabolic pathways.
! Enzyme synthesis is switched on by
the nutrient the pathway uses.
Positive control of a regulatory system occurs only if an activator molecule interacts directly with the genome to turn on transcription.
CAP (catabolite activator protein) = A protein that binds within an operon’s promoter region and enhances the promoter's affinity for RNA polymerase.
! When glucose is missing, the cell accumulates cyclic AMP ( cAMP ), a
nucleotide derived from ATP. cAMP activates CAP so that it can bind to the lac promoter.
! When glucose concentration rises, glucose catabolism decreases the
intracellular concentration of cAMP. Thus, cAMP releases CAP.
Catabolite repression = Repression of a variety of unrelated catabolic enzymes when cells are grown in a medium containing glucose.
Enhancer/Silencer = Regulatory regions on eukaryotic DNA that bind activator/repressor proteins controlling single gene expression.
