L11 - Regulation of gene expression - mechanisms Flashcards
What is the structure of lac operon?
Lac operon encodes 3 structural genes that are required for the utilisation of lactose - lacZ, lacY and lacA.
When is lac operon expressed?
only when the cells need to metabolise lactose.
and lactose is present
What regulatory gene controls the expression of lac operon?
Expression of the lac operon is controlled by the regulatory gene lacI.
What is the name of the lac operon inducer?
allolactose - a lactose isomer with a l,6 B-galactosidic linkage. Allolactose is generated by B-galactosidase in a side reaction and can be metabolised.
The lacI repressor binds both DNA and inducer. What is the lacI repressor and how does it work?
The lacI repressor is a homotetramer with 3 distinct regions: a tetramerisation domain, a core domain that binds the inducer and a head domain that binds the operator sequence.
When bound to inducer, the repressor alters shape and cannot bind DNA - it binds DNA and inducer in a mutually exclusive manner. This is an example of an allosteric interaction.
The repressor is expressed constitutively – lac expression is inhibited in the absence of lactose
What happens when the lacI repressor binds to the operator sequence?
the DNA binding sites of each subunit are aligned. This is thought to cause the DNA to twist.
This does block transcriptional activity so upon induction, the repressor will be released from the promoter region and transcription can start immediately.
Why is lac operon regulated by catabolite repression?
In the presence of glucose, expression of genes required for lactose is repressed.
When limiting glucose and lactose are both present in the medium, lactose is only metabolized after glucose is exhausted. This causes a diauxic growth curve.
A catabolite of glucose represses expression of the lac operon.
Expression of the lac operon requires both the presence of lactose and the absence of glucose
