Transcription Regulation Notes

Control of gene expression: Transcritpion

Regulation of gene expression

-some genes are expressed in all cells at constant levels- house keeping genes

-other genes vary in their expression- the control is by

Bacteria

-Chromosome of E coli : single circular DNA

-Expression of gene is regulated according to availability of food

-E.g 5 E.coli genes code for enzymes that synthesise amino acid tryptophan-genes are arranged as a single operon: Operon is a group of genes with related function which are under the control of a single promoter. Operons are only found in prokaryotes.

-genes are adjacent to one another and are transcribed from a single promoter as one long Mrna

-Promoter- specific DNA sequence that directs RNA polymerase to bind to DNA, open DNA double helix, begin synthesising RNA

-When tryptophan is present in the growth medium and enters the cells, the promoter is switched off. Within the promoter is a regulatr element called an operator-regulatory DNA that is recognised by repressor protein-binding of the protein prevents RNA polymerase binding to promoter region

-Gene regulatory proteins that switch genes off are transcriptional repressors

-Transcriptional activators: bind to a nearby site on DNA, contact the RNA polymerase that results in an increase in the probability that transcript will be initiated.

2) DNA looping: gene regulation

-Lac operon codes for proteins required to transport lactose into cell and break it down

-Operon is under both negative and positive transcriptional controls,Lac repressor protein and CAP (Catbolite activator protein)

• expression of the lac operon allows bacteria to use lactose as an energy source. In the absence of lactose, the lac repressor protein binds to the operator and blocks expression

• when an inducer molecule is present, in this case- allolactose (the inducer molecule of lactose)- it binds to the repressor protein and changes its shape so it can no longer bind to the operator and so RNA polymerase can transcribe genes involved in bacterial lactose metabolism

-Lac repressor binds tow operators- resulting in lloping of the DNA

3) Interchangeable RNA polymerase subunits

-Sigma subunit is required for bacterial RNA polymerase to recognise a promoter

-Bacteria produce a whole range of sigma subunits, each of which interact with RNA polymerase core and direct it to a different set of promoters

-This allows large set of genes to be turned off and a new set to be turned on simply by replacing one sigma subunit with another

Eukaryotic

-Genetic switches in bacteria respond to one or few signals, eukaryotes dozens of signlas converge on a single prometer, integration of different signals produce the appropriate level of Mrna

Key differences

a) RNA polymerase 2 requires 5 transcription factors, bacterial RNA polymerase needs only a single transcription factor- sigma subunit

The stepwise assembly of general transcription factors at eukaryotic promoter-provides multiple steps at which gene transcription can be controlled in response to gene regulatory proteins

b) Eukaryotic genes have no operons- so each gene is transcribed individually

c) Bacteria are controlled by a few regulatory proteins but eukaryotic hundreds of gene regulatory proteins

d) Pakaging of DNA into chromatin provides additional transcriptional regulation

Eukaryotic gene control region: Promoter plus regulatory DNA sequences

-Gene control region is the whole expandse of DNA involved in regulating and initiating transcription of a gene

-Promoter-general transcription factors and polymerase assemble

-Regualtory sequences are where gene regulatory proteins bind and control the rate of assembly processes at the promoter. It is the gene regulatory proteins that allow genes of an organism to be turned on or off individually

i) Gene activator proteins: promote assembly of RNA polymerase and general transcription factors at...

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