Question

Explain how the lac operon is subject to positive and negative control.

Answer 1

The lac operon consists of three structural genes -- lacZ, lacY and lacA. Besides these structural genes, it also has a promoter, a terminator , a regulator and an operator.

LacZ encodes B- galactosidase, an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose.

When a repressor protein binds the operator or control region upstrem of the lac operon, it prevents transcription. When lactose is present outside the cell, it crosses the cell membrane and acts as an inducer of the operon. CAP or the Catabolite Activator Protein, promotes RNA polymerase transcription of genes leading to an increase of lac operon expression.

So, negative control is carried out by Lac repressor and positive control is done by CAP.

When glucose is present in high concentration, the cyclic Adenosine Monophosphate [ cAMP] concentration is low.As the concentration of glucose increases, the cAMP concentration increases.The high concentration of cAMP is necessary for the activation of the lac operon.

CAP forms a complex with cAMP, and it is this complex which is able to bind to the CAP site of the operon. The DNA bound CAP is then able to interact physically with RNA polymerase and essentially increase the affinity of RNA polymerase for the lac promoter. In this way, the catabolite repression system helps to the selective activation of the lac operon.

The lac operon is a negatively controlled inducible operon, where the inducer molecule is an allolactose . In negative repressible operons, transcription of the operon normally takes place. Repressor proteins are produced by regulator genes but are unable to bind to the operator in their normal conformation.

The repressor has two different domains-- one that binds to the DNA and the other that binds to the inducer and other subunits called the 'core'. Binding of inducer to the 'core' causes an allosteric shift in the repressor so that the domain binding to the DNA is no longer able to form a high affinity complex with it [ the DNA] . The repressor can dissociate, that is, it can go to any other non specific sites.