IN PROKARYOTES (LAC OPERON)
In bacteria, expression of gene is controlled at transcription level. Operon model of control of gene expression was described by
Francois Jacob and Jacques Monod in 1961. The entire stretch of DNA required for production of an enzyme is called Operon. The Operon which controls the concentration of lactose in bacteria is called Lac Operon. Lac operon is involved in both negative and
positive control of gene expression.
(a) Negative control of Lac operon
There is following mechanism of negative control of lac Operon in E. coli bacteria. E. coli bacteria live in the gut of man. Lactose sugar is present in milk. Lactose is also available to bacteria if host (man) drinks milk. Bacteria break down this compound in its body for energy requirement. An enzyme p- galactosidase is required for the
breakdown of lactose. If lactose is available to bacteria, it synthesizes this enzyme within 15 minutes. The gene of pgalactosidasi is part of operon called Lac operon. It also includes two other st uctural genes required for the metabolism of lactose. The entire opo:n is under the control of single promoter. Another
regulatory gen,!so present outside the lac operon. It codes for a
repressor protein specific metabolite inducer inactivates the
repressor. The indu. er L; allielactuse for lactose. Allolactose is an isomer of lactose.
- Absence of lactose: The allolactose is in inactive state in the absence of lactose. Therefore, repressor attach with the operator. Hence, lac operon remains non-functional.
- Presence of lactose: If lactose is added, allolactose become
active. It binds to lac repressor and alters its configuration. Now repressor cannot attach with the promoter. Now RNA polymerase can attaches \kith the promoter. Transcription starts and mRNA is produced. This mRNA is transported to ribosome. Ribosome synthesizes 13- galactosidase. This enzyme breaks the lactose sugar.
(b) Positive Control of Lac operon
The first preference of bacteria for energy requirement is glucose. It uses lactose only in the absence of glucose. Positive control of gene expression in lac operon occurs by CAP (cataholite activating protein). CAP accelerates transcription of lac operon. CAP attaches at the CAP binding site of promoter. It helps in attachment of RNA polymerase at promoter. Thus CAP stimulates the gene expression. So this mechanism is called positke regulation.
I. Absence glucose: CAP is sensitive to glucose requirement of
cell. There is shortage of ATP in bacteria in absence of glucose. Therefore, AMP changes in to Cyclic AMP (cAMP). The cAMP accumulates in the cell. CAP has binding site for binding site for cAMP. Thus CAP-cAMP complex is formed. It attaches with the promoter and stimulates the transcription of mRNA of 0- galactosidase. Now this enzyme starts breaking lactose.
2. Presence of glucose: Now if glucose is added, the concentration
of cAMP falls. Thus CAP molecule detach from the lac operon
and transcription stop. Thus although lactose is present, but glucose is available to bacteria. So it does not waste energy in
breaking of lactose. Thus synthesis of [3- galactosidase is stopped.