Regulation of Gene Expression Prokaryotes and Eukaryotes

Regulation of Gene Expression A cell contains the entire genome of an organism– ALL the DNA. Gene expression = transcribing and translating the gene Regulation allows an organism to selectively transcribe (and then translate) only the genes it needs to. Genes expressed depend on the type of cell the particular needs of the cell at that time.

Gene Regulation in Prokaryotes Prokaryotes organize their genome into operons Operon = a group of related genes One promoter sequence at the very beginning All of the genes will be transcribed together (in one long strand of RNA.

Question… What is the benefit of organizing the genome into operons? It’s more efficient – transcribe everything you need for a process at once.

Repressible Operon: Trp Operon Repressible Operon = Operon that is usually “ON” but can be inhibited The Trp Operon example of a repressible operon Genes that code for enzymes needed to make the amino acid tryptophan

TrpR Gene TrpR gene is the regulatory gene for the Trp operon Found somewhere else on the genome NOT part of the Trp operon TrpR gene codes for a protein = TrpR repressor TrpR gene is transcribed and translated separately from the Trp operon genes.

TrpR Repressor Repressor protein is translated in an inactive form Tryptophan is called a corepressor When tryptophan binds to the TrpR repressor, it changes it into the active form

Operator Region There is also an operator region of DNA in the Trp Operon Just after the promoter region The TrpR Repressor can bind to the operator if it’s in the active form

Trp Operon Transcription is “ON” Occurs when there is no tryptophan available to the cell. Repressor is in inactive form (due to the absence of tryptophan) RNA Polymerase is able to bind to promoter and transcribe the genes.

Trp Operon Transcription is “OFF” Occurs when tryptophan is available Tryptophan binds to the TrpR repressor  converts it to active form TrpR protein binds to operator  blocks RNA Polymerase  no transcription

Question… Under what conditions would you expect the trp operon to go from “OFF” to “ON” again? When there is no longer tryptophan available– all of it has been used up

Inducible Operon: Lac Operon Inducible operon = operon is usually “OFF” but can be stimulated/activated Lac Operon Example of an inducible operon Genes code for enzymes that break down lactose

LacI gene LacI gene is the regulatory gene for the lac operon Found somewhere else on the genome NOT part of the lac operon LacI gene codes for a protein = lacI repressor LacI gene is transcribed and translated separately from the lac operon genes.

LacI Repressor The lacI repressor protein is translated into an active form When the lacI repressor is bound by lactose (also called allolactose) it becomes inactive Lactose is the inducer

Lac Operon Transcription is “OFF” When there is no lactose that needs to be digested lacI repressor is in active form  binds to operator  blocks RNA Polymerase  no transcription

Lac Operon Transcription is “ON” When there is lactose that needs to be digested Lactose binds to lacI repressor  inactivates it RNA Polymerase is able to bind to promoter  transcribe genes

Do all operons have operator regions? NO There are some genes that always need to be transcribed  they do not need to have operators to regulate them in this manner. Ex. genes that participate in cellular respiration