Describe the positive and negative control of lac operon.

The lac operon is responsible for the metabolism of lactose and is also induced by lactose.

Negative control of lac operon:

The lac I gene of lac operon encodes a repressor which, in the absence of lactose binds to the operator (o) and blocks the transcription of the three structural genes; 

Z ; beta-galactosidase

y; lactose permease

a; thiogalactosidase transacetylase

Lactose induces expression of the operon by binding to the repressor, which prevents the repressor from binding to the operator. The lac operator overlaps the promoter and so the repressor bound to the operator physically prevents RNA Pol from binding to the promoter. (Repressor-operator interactions).

Allolactose then acts as an inducer and binds to the lac repressor, leading to its dissociation from DNA and hence, the production of lac mRNA (and the 3 enzymes).

Positive control of the lac operon:

It is also called the catabolite control of lac operon. The initiation of lac mRNA synthesis is also regulated by the concentration of glucose. No lac mRNA is made in the presence of glucose.

Cyclic AMP (cAMP) is synthesized by the enzyme adenylate cyclase and its concentration is regulated indirectly by glucose. The entry of glucose in the cell makes the intracellular concentration of cAMP quite low. Under starvation conditions, the cAMP concentration in bacterial cells is high. It is cAMP that links the lac operon the activity with the intracellular concentration of glucose.

E.coli contains a protein called CRP (cAMP receptor protein). CRP and cAMP bind together to form a complex (denoted cAMP-CRP) which is the regulatory element in the lac system. The cAMP-CRP complex binds to a specific sequence called an activation site in the promoter region for transcription of the lac-operon to occur. Thus, the cAMP-CRP complex is a positive regulator (activator) of the lac operon.