Distal control elements , enhancers , may be thousands of nucleotides away from the promoter or even downstream of the gene or within an intron. A given gene may have multiple enhancers, each active at a different time or in a different cell type or location in the organism. Interactions between enhancers and specific transcription factors called activators or repressors are important in controlling gene expression. An activator is a protein that binds to an enhancer to stimulate transcription of a gene. Protein-mediated bending of DNA brings bound activators in contact with a group of mediator proteins that interact with proteins at the promoter. ○ These interactions help assemble and position the initiation complex on the promoter. Hundreds of transcription activators have been discovered in eukaryotes. ○ Two common structural elements characterize a large number of activator proteins: a DNA-
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