Module 04 - Section 06 Flashcards
Regulation of Gene expression
What was François Jacob and Jacques Monod’s main contribution to molecular biology?
Some genes encode protein with the sole function of regulating the expression of other genes
Why is it important to spend time learning about how E.Coli metabolize lactose?
Bacteria are simple model to study positive and negative gene regulation
Because they are haploid the effects of gene mutations are completely unmasked (easy to see the impact)
What is an Operon?
A unit of genetic expression consisting of one or more cotranscribed genes and the operator and promoter sequences that regulate their transcription
What are 2 defining characteristics of bacterial operons?
(1) A set of genes (A,B, and C) transcribed as a single mRNA
(2) Adjacent regulatory regions that coordinately control the expression of the operon genes. These regions can contain the same promoter, activator, and repressors regions as the genes
What is lac operon?
a polycistronic mRNA which contain two regulatory regions (lacI and lacO) and three genes that are sometimes referred to collectively as the lac genes (lacZ, lacY, lacA)
What does lacI do?
lac repressor protein, which interacts with lacO to regulate transcription – Repressor is transcribed separately from the rest of the operon and is “always on”
What does lacO do?
Does not encore for a gene product, but instead interacts with lacI to regulate transcription
What does lacZ do?
Codes for the protein beta-galactosidase, which catalyzes cleavage of lactose into its components (glucose and galactose)
What does lacY do?
Codes for the galatoside permease protein which inserts into the bacterial plasma membrane and imports lactose into the cell
What does lacA do?
Codes for the thiogalactoside transacetylase which modifies toxic galactosides that are imported along with lactose, facilitating their removal from cell
Describe the experiment of Jacob and Monod that used lac Operon mutant E. coli to understand the mechanisms of gene regulation
Intro: They created partial diploid introducing a mutant lac operon with a wild type one
Results: Wild-type lacI allele was able to make up for the defect in the lacI mutant (had normal regulation)
Conclusion: lacI locus produced a diffusible product that could act on any DNA molecule (it regulated the lacI mutant)
Describe the experiment of Jacob and Monod that examined the role of the lap operator
Intro: they introduced a defective lacO mutant in E.coli that also had a working wild type
Results: Wild-type was not capable of rescuing the defect in mutant lacO
Conclusions: lacO did not produce a diffusable substance – thus it was a receiver
Describe the experiment of Jacob and Monod that examined the role of lac Operator part dos
Intro: combined a mutant lacZ with an operon carrying mutant lacO and lacY and wanted to know which operon would give rise to a gene product
Results: in the absence of lactose this diploid construction produce galactosidase
Conclusion:
confirmed lacI encodes a diffusable molecule that represses gene expression whereas lacO controls only the expression of lac operon genes to which it is connected
Describe the structure of lac Repressor
Homotetrameric DNA binding protein it has 2 dimers bound together at the end which is opposite to the DNA binding region
What are the three regions of lacO?
(1) O1 - 3’ of promoter region
(2) O2 - in lacZ gene
(3) O3 - 5’ of promoter
Where does the lac Repressor bind?
ALWAYS to O1 and then either O2 or O3, which ties DNA in a loop blocking transcription of the operon by RNA Pol
What is an allosteric effector?
usually a small molecule or another protein that binds the repressor and causes a conformational change that results in an increase or decrease in transcription
How does an effector “activate” the repressor?
Effector binds to the repressor and induces a conformational change that results in dissociate of the repressor from its binding site on the DNA, allowing transcription to proceed
describe the sequence of operator binding sites
they are inverted sequence of each other
How does an effector “inactivate” the repressor?
Interaction of an inactivating repressor and an effector moelcule causes the repressor to bind to DNA, shutting down transcription
