Regulation of Gene Expression SBI4U1 Regulation of Gene Expression

Gene Regulation: control of the level of gene expression -whether a gene is active or inactive -determines the level of activity and the amount of protein that is available. Constitutive Genes: genes that are always active and expressed at constant levels “ housekeeping genes”

Transcription * most common during initiation Translation Regulation of gene expression in prokaryotes occurs at three different levels Transcription * most common during initiation Translation After protein has been synthesized

Recall? Promoter region is where RNA polymerase binds to DNA to initiate trancription What is an Operon? A cluster of genes under the control of 1 promoter -these genes are all transcribed together Let’s use E. Coli as an example…they use different sugars as a source of energy

Genes that encode the enzymes needed to break down the sugar lactose The “Lac Operon” Genes that encode the enzymes needed to break down the sugar lactose Composed of 2 regions: regulatory region & coding region Enzymes which break down lactose Promoter Operator Z Y A Controls whether gene in transcribed or not RNA polymerase binds here to start making mRNA copy

Here’s how it works… In the absence of lactose, the lac repressor protein binds to the operator, preventing RNA polymerase from binding to the promoter to initiate transcription. In the presence of lactose, an activator binds to the repressor so it can no longer bind to the operator. Transcription of the genes can therefore occur.

If tryptophan is present; no transcription The “Trp Operon” If tryptophan is present; no transcription Genes synthesize tryptophan. The operon is transcribed until sufficient tryptophan is present. Promoter Operator E D C B A Trp repressor binds to promoter and inhibits transcription

“Trp Operon” contd.. The trp (tryptophan) operon, however, is normally active until a repressor turns it off. It contains:  a coding region with five genes for enzymes required for tryptophan synthesis a regulatory region with a promoter and an operator

The trp operon contains five genes that are involved in the synthesis of tryptophan. This operon is normally transcribed until the cell has sufficient tryptophan. Then the trp repressor binds to the promoter and inhibits transcription.

Regulation in Eukaryotes 5 levels of regulation: Pre-transcriptional Transcriptional Post-transcriptional Translational Post-translational

Pre-Transcriptional/Transcriptional DNA in highly condensed chromatin is not transcribed Chromatin is physical barrier to proteins needed to synthesize pre-mRNA Can be loosened so transcription can occur via chromatin remodelling complexes Transcription factors are a set of proteins needed to RNA polymerase to bind to promoter Activators bind to transcription factors, RNA polymerase, enhancers

Post-Transcriptional/Translation mRNA level 5’ cap/poly-A tail may not be added mRNA does not leave nucleus Or is degraded in cell Small RNA molecules control gene expression via RNA interference Micro RNA (miRNA) and small interfering RNA (siRNA) turn off gene expression

Post-Translational Polypeptide level They must be activated by modifications E.g. Insulin is folded into 3D structure Removal of amino acids activates it Leaves two polypeptide chains Regulating length of protein in cell also regulates the gene Adding a chain of molecules can signal protein degradation

Learning Expectations... What is gene regulation? Where does it occur in a cell? How do the Lac and Trp operons work? Compare/contrast Difference in gene regulation in prokaryotes and eukaryotes