What activates the Lac operon under low glucose conditions?

The activation of the Lac operon under low glucose conditions is primarily driven by the binding of the catabolite activator protein (CAP) to the promoter region of the operon. In the presence of low glucose, the concentration of cyclic AMP (cAMP) increases. cAMP then binds to CAP, forming a cAMP-CAP complex that enhances the binding of RNA polymerase to the promoter, facilitating transcription of the operon. This mechanism is a classic example of how cells prioritize energy sources; when glucose is scarce, the cell activates genes necessary for the metabolism of alternative sugars, such as lactose.

Understanding this process underscores the interplay between glucose levels and the regulation of the Lac operon. In low glucose conditions, the increase in cAMP ensures that CAP can effectively activate the transcription of lactose-utilization genes, enabling the cell to use lactose as an alternative energy source efficiently.