Question

Describe a model of prokaryotic gene regulation that was discussed in class, or a part of your readings for this semester. How are genes in this “model” turned on, turned off, or how the rate of transcription can be slowed down.

Answer 1

In prokaryotes transcription regulation is occurs through operon. An operon is consists of group of genes which is situlated contiguously on DNA and these genes are generally controlled by one promoter. An operon is normally consists of a promoter, operator and structural genes.

An example of prokaryotic operon is Lac operon of E. coli.

• Lac operon is consists of a promoter, operotor, structiral genes, such as lac Z, lacY , lacA and a terminator. Additionally a repessor gene known as lacI is also present. These structural genes encodes three enzymes β-galactosidase (lacZ), Beta-galactoside permease (LacY) and β-galactoside transacetylase (LacA) which are involved in lactose metabolism and breaks down lactose to glucose and galactose.

• The lac operon is inducible and becomes activated or inactivated depending on the presence of Lactose in E.coli environment. In absence of lactose, a lac repressor binds to the operator of lac operon which prevent the RNA polymerase to transcribe the structural genes, making the system switched off. Whereas, when lactose is present, it convereted to allolactose which can act like an inducer and binds to the repressor, changes its shape thus preventing the repessor to bind to the promoter. As repressror is removed RNA polymerase can transcribe the structural genes .This is known as switching on of the lac operon.

• Additionally, the expression of lac genes can be reduced in presence of glucose. As glucose is easy to break down than lactose, the requirement of lactose metabolism reduces greatly. This effect of glucose in known as catabolite repression.