Promoters are regions of DNA that are located near the transcription start sites of genes. They play a key role in initiating transcription of a particular gene and regulating where and when the gene of interest is expressed. In eukaryotes, the primary function of promoters is to facilitate binding of transcription factors and RNA polymerase for gene transcription. In prokaryotes, the primary function of promoters is to facilitate binding through the RNA polymerase and associated sigma factor.