Question

(a) Draw the lac operon.

(b) What role does the lacI gene product play in expression of β-galacatosidase?

(c) How does the lac repressor prevent binding of RNA polymerase?

(d) What role does β-galactosidase play in regulation of the lac operon?

(e) What is the purpose of the lacY gene product?

Answer 1

b) In the absence of an indcer like IPTG, the lac I gene is trascribed and then translated to produced lac repressor protein. Four identical lac repressor momomers form the active tetramer which tightly bind to the lac operator site, O_lac and prevents transcription of the lacZ, lacY and lacA gene. As beta galactosidase transcribed from the lacZ gene, expression of beta galactosidase is not occured in this case.

c) The operator site is present in between the promoter and the structurel gene.The O_lac sequence is palindromic. The symmetry of the operator site is matched by the symetry of the repressor tetramer. Therefore lac repressor tetramer bind tightly to the lac operator site.O_lac. and thus preventing binding of RNA polymerase.

d) High level tanscription of the lac operon requires the presnce of a specific activator proein CAP (Catabolite Activator Protein) or CRP (cAMP Receptor Protein). This protein along with cAMP or cyclic AMP forms CRP-cAMP complex whcih binds to the lac promoter site and increases binding of RNA polymerase and also the transcription of the lac operon.

The binding of CRP protein to the lac promoter depends on the carbon source available to the bacterium. When glucose is present E.coli does not need to use lactose as carbon source so the lac operator does not need to active. In the presence of glucose the intracellular level of cAMP falls and CRP can't bind to the promoter. Threfore glucose play a crucial role in regulation of lac operon.

In E.coli, the gene of beta galactosidase lacZ is present as a part of the inducible system of the lac operon which is activated in the presence of lactose when glucose level is low. It cleaves the disaccharide lactose into glucose and galactose.Thus beta galactosidase play a crucial role in regulation of the lac operon.

e) lacY encodes lactose permease. It is a transmembrane symporter that  pumps β-galactosides including lactose into the cell using a proton gradient in the same direction.