Question

Compare/Contrast the Lac and Ara operons.

Answer 1

LAC OPERON	ARA OPERON
Metabolism of lactose sugar in E.coli in the absence of glucose in the external environment.	Metabolism of Arabinose sugar in the absence of glucose in the external environment
Three genes are responsible for the metabolism of lactose in the lac operon: lacZ lacY lacA	Three genes are responsible for the metabolism of arabinose in the ara operon: Ara B Ara A Ara D
The three genes lac Z, lac Y and lac A code for three different enzymes which are β-galactosidase, β-galactoside permease, and galactoside acetyltransferase respectively.	The three genes Ara B, Ara A, and Ara D code for the enzymes ribulokinase, L-arabinose isomerase, and L-ribulose-5-phosphate-4-epimerase respectively.
Activation of lac operon occurs when allolactose binds to the lac repressor.	Ara C gene, which acts as a repressor, when bound to arabinose activates the ara operon.
RNA polymerase with the help of a protein called as CAP or the Catabolite Activator Protein (which in turn needs cAMP to be activated) binds to the lac operon promoter and begins the transcription process.	The transcription of ara operon by RNA polymerase requires the activation of CAP-cAMP at the CAP site and the binding of arabinose to Ara C protein in the operator region.
When glucose level increases cAMP decreases, thereby activating the lac repressor and turning off the lac operon. This is the negative regulation in the lac operon.	Negative regulation in ara operon is by the Ara C protein, which acts as a repressor and creates a DNA loop that inhibits the RNA polymerase to undergo transcription.