Regulation of Gene Expression Prokaryotes and Eukaryotes Regulation

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

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

Trp. R Repressor • Repressor protein is translated in an inactive form • Tryptophan is called a corepressor – When tryptophan binds to the Trp. R 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 Trp. R 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 Trp. R repressor converts it to active form – Trp. R 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

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

Lac. I Repressor • The lac. I repressor protein is translated into an active form • When the lac. I 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 – lac. I 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 lac. I 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