Question

How many promoters can an enhancer activate at a given moment? Do these promoters need to be on the same chromosome? How can one determine which promoter is being activated by a particular enhancer?

Answer 1

An enhancer can activate a multiple promoters at one moment. However they have to be present on the same gene.

Enhancers are short nucleotide sequences which influence the rate of transcription of specific genes. Enhancers are cis-acting which means that they can influence only genes on the same DNA molecule. These sequences need not be present in the vicinity of the gene, they can be located upstream of a gene or downstream of a gene, or may be thousands of nucleotides away , within the coding region of the gene..Since they cannot increase the transcription of genes on a different chromosome, so the promoters have to be on the same chromosome.

When a DNA-binding protein binds to an enhancer, it alters the shape of the DNA.Change in shape  allows the interaction between the activators and the transcription factors bound to the enhancer and promoter region respectively and with the RNA polymerase to occur.

To identify the enhancer and promoter pair one can perform yeat two hybrid system. It is molecular biology technique used to discover protein–protein interactions and protein–DNA interactions.