Lecture 11 Regulation of Gene Expression 1 Understand the structure of the lac operon and the functions of the promoter repressor operator and inducer in controlling the production of polycistronic lac mRNA especially the activation of CAP and inhibition of repressor Some bacteria such as e coli use glucose as a source of both energy and carbon atoms When there is little to no glucose available e coli can use lactose for carbons instead Three genes lacZ lacY and lacA are transcribed from a promoter P that produces a single mRNA from which three proteins are translated An operon is a cluster of genes LacZ Y and A that act as a switch to toggle a system or action on and off The Lactose operon of e coli is regulated by 2 things operator and CAP binding site Operator is by default suppressed by the lac repressor LacL produces repressor which inhibits transcription by binding to the lac operator O Something called an inducer can inhibit the repressor binding to the operator which stops the repressor and allows the operator to start The CAP binding site is activated by a catabolite activator protein which stimulates the transcription of the lactose gene when it is bound to cAMP In this system glucose makes CAp and regulates its activity The repressor is produced by LacL The presence of a positive regulator cAMP regulates a process to go A negative regulator s presence repressor regulates a system to stop The lactose operon only functions when the glucose is not available and lactose is When glucose is present adenylyl cyclase is inhibited and then cAMP is not made For the lactose operon to be turned on cAMP must bind to CAP to eliminate glucose And allolactose must bind to the repressor protein which causes a conformational change and prevents it from binding to the operator This whole cycle is an example of gene expression being regulated at the transcription level in prokaryotic cells 2 What are the cis acting elements and trans acting factors involved in eukaryotic transcription Understand the difference between promoter and enhancer Cis acting elements are DNA sequences that regulate the expression of a gene on the same molecule of DNA Trans acting elements or factors are proteins that recognize cis acting elements and regulate RNA synthesis These are also called transcription factors There are two kinds of transcription factors basal and tissue specific Enhancers found either upstream or downstream are short nucleotide sequences that enhance the transcription rate in the genome Enhancers are small and carry transcription along Promoters found only upstream are fairly large nucleotide sequences that initiate the process of transcription Promoters are big and get the transcription process started 3 Understand how hormones regulate gene expression lipid soluble and water soluble hormones Gene expression is regulated by hormones and there are two kinds of hormones lipid soluble and water soluble Hormone s role is to take signals from one place to another Extracellular into the cells and this can happen in gene expression Hormones are produced outside of the cells and are carried into the cell cytosol or nucleus Lipid soluble hormones and hydrophobic by definition These are able to pass directly through a membrane into the cell Water soluble hormones on the other hand must pass through a membrane acceptor to enter the cell because they cannot pass through the lipid membrane alone Lipid soluble hormones pass through the membrane into the cell and bind directly to the hormone receptors and enter the nucleus Hormone receptors bind to specific areas of the DNA called hormone response elements of HRE this activates gene expression Nuclear hormone receptors are transcription factors and have two domains the DNA binding domains and the ligand binding domain Water soluble Hormones have a first and second messenger The first messenger is the hormone itself and after it enters the cell it somehow activates the second messenger such as cAMP which goes on to bind to kinases which can activate genes for cell growth or proliferation The second messenger can be cAMP cGMP or calcium inside cell 4 What is mRNA editing How does mRNA editing regulate gene expression of mRNA editing is a way to change the encoded protein sequence so that it differs from ApoB 48 original DNA ApoB 48 is an apolipoprotein part of the outside layer of chylomicrons which are full of lipid droplets Compounds like Apo B 48 phospholipids and free cholesterol make up a thin layer coating the chylomicron TG RNA editing can deaminate a cytidine to a uridine by using cytidine deaminase C to U within the apoprotein B gene This converts it a codon from CAA codes for an AA to UAA which is a stop codon This is a nonsense mutation and will stop the transcription prematurely shortening the original RNA and producing Apo B 100 instead of ApoB 48 5 How does the iron level regulate the expression of transferrin receptor and ferritin via translation or mRNA stability This is an example of gene regulation at the mRNA level In iron metabolism when there is a normal amount of iron available in a system it will bind to the transferrin receptor TfR This is a carrier protein for transferrin which is needed to import the iron into the cell This system is regulated by the intracellular iron concentration The transferrin imports iron by internalizing the transferrin iron complex through receptor mediated endocytosis In short Transferrin absorbs iron into cells when normal amounts of iron are available in a system Low iron high production of transferrin Ferritin is a protein that stores iron long term and releases it when the body needs it This system kicks into gear when excess iron is available in a system Production of the TfR and ferritin is regulated at the mRNA level by iron regulatory proteins or IRPs which bind to iron response elements IREs on the 3 of TfR and the 5 of ferritin on the untranslated regions of their respective mRNAs The binding of the IRPs too the IRE of TfR on the 3 of mRNA stabilizes mRNA which facilitates translation The binding of the IRPs to the IRE of ferritin on the 5 end of mRNA inhibits the translation Maybe this is because it inhibits it so it can leave it in storage The production of the TfR and ferritin is regulated at the mRNA level by iron regulatory proteins which bind to IREs 6 What is epigenetics How does reversible acetylation of histone tails on chromatin regulate gene transcription Epigenetics is a study of