Which of the following is a common promoter for multiple genes?

The correct answer is that lacP, the promoter for the lac operon, is a common promoter for multiple genes. In the context of prokaryotic gene regulation, particularly in the case of the lac operon in Escherichia coli, lacP serves as the initiation site for transcription of the genes responsible for lactose metabolism: lacZ, lacY, and lacA. This means that when lacP is activated, it drives the expression of all three genes simultaneously, allowing for the coordinated regulation of genes that work together in a specific metabolic pathway.

This promoter is a key component of how cells efficiently regulate gene expression in response to environmental changes, particularly the presence of lactose. Such operons, characterized by a single promoter regulating multiple genes, facilitate rapid shifts in gene expression, which is essential for survival in varying nutrient conditions.

While the other choices represent important elements associated with gene regulation, they do not function as promoters. Allolactose is a metabolite of lactose that acts as an inducer for the lac operon, CAP-cAMP complex is a regulator that enhances the transcription of operons in low glucose conditions, and an inducer is a molecule that initiates gene expression but does not directly serve as a promoter. Thus