2) Control of Transcription Flashcards
Why is it important that the amount a gene is transcribed and translated be varied?
Not all proteins are required in the same amounts or at the same time
Switching transcription of genes on and off when needed SAVES ENERGY
E.coli: Only needs to transcribe genes needed to make proteins to use the food substrates
How do prokaryotes control the amount of transcription?
1) Cell can control which genes are transcribed (where in the DNA transcription starts and stops)
2) Can control the rate and amount of transcription
What are the 3 different types of control mechanisms used by bacteria? (Names Only)
1) Operons: Coordinate control of gene groups
2) Alternative sigma factors: Decides which genes are transcribed
3) Regulating transcription termination: Decides whether additional genes are transcribed
What are operons?
What are the 2 types?
Genes which have related function in bacteria are grouped together and are regulated in a coordinate fashion= Operon
Controlled by a shared promoter
Regulatory proteins bind to the DNA of the promoter and its operator to regulate expression
Example: 9 genes making the 9 enzymes which synthesise histidine= Operon
Can be controlled by positive or negative mechanisms
2 types:
1) Catabolic (e.g. lac operon)
2) Biosynthetic (e.g. Trp operon)
What is the difference between positive or negative control?
The DNA-binding protein that binds the promoter can be a positive or negative regulator
Positive= Transcription is switched ON
Negative= Transcription is switched OFF
What the difference between catabolic and biosynthetic operons?
Catabolic: When the operon is involved in breaking down substrates. Produce molecules they can use for extracting energy
Biosynthetic: Synthesising new molecules they require for life
Where are catabolic operons used?
Control expression of enzymes used in sugar metabolism and utilisation
What happens to catabolic operons when glucose is available?
Glucose= Preferred sugar
Do not needed to switch on expression of enzymes which break down other compounds and release sugars= Transcription= OFF, normally OFF
These operons (such as Lactose (lac) and arabinose (are) are ONLY switched on when their sugar is present
What does the lactose operon do?
Lac gene: Codes for the enzyme B-galactosidase which breaks down the sugar lactose= Cleaves the beta 1,4 linkage
Lactose: Disaccharide made up of 2 monosaccharide sugars joined together= Glucose + Galactose= E.coli can only use the monosaccharides
What are the different genes of the lac operon?
lacZ, lacY and lacA= Lac operon AND also lacI
Structural genes and are transcribed from a single promoter into polycistronic mRNA
What is the difference between structural and regulatory genes?
Structural genes: Code of enzymes or others that actually DO the work
Regulatory genes: Control the operon
What is the regulatory gene for the lac operon?
LacI= Gene that produces the regulatory protein. It has its own promoter and terminator
Product of lacI regulates expression of lacZYA
If its function is disrupted, all of the other 3 genes are expressed ALL the time
It encodes a REPRESSOR
What does lacZ do?
Encodes the enzyme beta-galactosidase which is very stable
Proof experiments: Always monitor lacZ as it is the easiest to detect= Only need to monitor one of them
Able to measure beta galactosidase activity easily= turns blue on X-gal
What does lacY do?
Encodes a permease= Transports lactose into the bacteria as it cannot diffuse through the plasma membrane by itself
What does lacA do?
Encodes an enzyme transacetylase= Add acetyl groups to toxic sugars= Makes them harmless
(Although no one really knows its function…)