OPERONS – GENES THAT CODE FOR ENZYMES ON THE SAME PATHWAY ARE REGULATED AS A GROUP

Operons  Operons – set of genes coding for enzymes on one metabolic pathway controlled by the same operator  Operator- “on-off” switch for operons that includes regulatory gene (repressor or activators) and promoter region  Bacteria need to be able to turn genes on or off in response to environmental conditions – don’t want to waste energy and material transcribing unnecessary products

Fig. 18-UN2 Promoter Genes Genes not expressed Inactive repressor: no corepressor present Corepressor Active repressor: corepressor bound Genes expressed Operator ON-OFF SWITCH IS A PROTEIN REPRESSOR THAT BINDS TO OR COMES OFF DNA IN RESPONSE TO THE BINDING OF A SMALL SIGNAL MOLECULE TO REPRESSOR; WHEN REPRESSOR IS BOUND, RNA POLYMERASE CANNOT TRANSCRIBE GENES.

Regulation of Operons  Regulation may be: A) Repressible –default is on, genes can be turned off (repressed) B) Inducible- default is off, genes can be turned on (induced)

Repressible vs. Inducible Repressible  Example: Trp operator  Default on – Trp is essential amino acid (ususally need to make)  Regulation controlled by amount of final product in pathway (synthesis or anabolic pathway)  Trp abundant – trp binds to repressor to turn operon off Inducible  Example: Lac operon  Default is off – glucose is preferred sugar source; only use lactose when no glucose is present (rare situation)  Regulation controlled by first reactant in pathway (catabolic or digestion pathway)  Glucose low, lactose high, remove repressor, activate operon

Link to Tryp operon animation

Fig. 18-3a Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on DNA mRNA 5 ProteinInactive repressor RNA polymerase Regulatory gene Promoter trp operon Genes of operon Operator Stop codon Start codon mRNA trpA 5 3 trpRtrpE trpD trpCtrpB AB CD E

Fig. 18-3b-1 (b) Tryptophan present, repressor active, operon off Tryptophan (corepressor) No RNA made Active repressor mRNA Protein DNA

Fig. 18-3b-2 (b) Tryptophan present, repressor active, operon off Tryptophan (corepressor) No RNA made Active repressor mRNA Protein DNA

Link to Tryp operon video

Fig Polypeptide subunits that make up enzymes for tryptophan synthesis (b) Tryptophan present, repressor active, operon off Tryptophan (corepressor) (a) Tryptophan absent, repressor inactive, operon on No RNA made Active repressor mRNA Protein DNA mRNA 5 Protein Inactive repressor RNA polymerase Regulatory gene Promoter trp operon Genes of operon Operator Stop codon Start codon mRNA trpA 5 3 trpR trpE trpD trpC trpB ABC D E SUMMARY TRYP OPERON

Operon Review Questions  1) What is an operon?  ANS: Set of genes coding for enzymes on one metabolic pathway controlled by operator  2) What is an operator?  ANS: “on-off” switch for operons that includes regulatory gene (repressor or activators) and promoter region

Operon Review Questions  3) Why would a bacterial cell need to control the expression of a gene based on environmental conditions?  ANS: Making proteins costs energy and material resources. Cells only make proteins when the proteins will help the cells survival.  4) In a repressible operon system is the default condition on or off?  ANS: DEFAULT IS ON, SYSTEM CAN BE TURNED OFF  5) In an inducible operon system is the default condition on or off?  ANS: DEFAULT IS OFF, SYSTEM CAN BE TURNED ON

Operon Review Questions  In a repressible operon, is the gene expression regulated by the first reactant in the pathway or the final product?  ANS: Final product  In an inducible operon, is the gene expression regulated by the first reactant in the pathway or the final product?  ANS: First reactant

Tryp Operon Review Questions  Under what conditions of tryptophan concentration, high or low, do most repressor molecules have tryptophan molecules bound?  ANS: High [Tryp ]  When tryptophan is bound to the repressor is the repressor in an inactive (doesn’t bond to DNA) or active (will bond to DNA) configuration.  ANS: active

Tryp Operon Review Questions  What segment of the DNA does the repressor bind to?  ANS: Operator  What is the effect on gene expression when the repressor binds to the operator?  ANS: The binding of the repressor blocks RNA polymerase from transcribing the genes. Therefore no new mRNA transcript will be made and thus protein product will decrease and eventually stop once existing mRNA is degraded.