A DNA region that initiates transcription of a particular gene.A promoter is a stretch of DNA that acts as a starting signal for reading a gene. It sits at the beginning of a gene, just before the sequence that actually codes for a protein or RNA molecule. When the cell needs that gene's product, proteins called transcription factors bind to the promoter, and an enzyme called RNA polymerase is recruited there to begin copying the DNA into RNA. If the promoter is blocked or mutated, the gene may be read too little, too much, or not at all. This makes promoters a central control point in biology. Every cell in your body carries roughly the same genes, but a liver cell and a nerve cell look and behave very differently because they use different sets of promoters. Promoters also explain how cells respond to signals: a hormone or stress signal can switch transcription factors on or off, and those factors then land on promoters to turn specific genes up or down. Because of this, promoters are a major focus in genetics, medicine, and biotechnology.
The concept of a promoter emerged from bacterial genetics in the 1960s, with key work by François Jacob, Jacques Monod, and others on the lac operon, and was later extended to eukaryotic genes.
The promoter contains short, recognizable sequences where transcription factors and RNA polymerase can dock. In many organisms, a common element is the TATA box, a run of thymine and adenine bases about 25 to 35 bases before the transcription start site. General transcription factors assemble on the promoter, then RNA polymerase binds and separates the two DNA strands. The polymerase moves along the DNA, building an RNA copy. Other nearby sequences, called enhancers, can loop over to contact the promoter and boost or dampen the rate of transcription. The strength of a promoter depends on its exact sequence and on which transcription factors are present in the cell at that moment.
Promoters are widely used in genetic engineering to control when and where a gene is expressed, for example in producing insulin in bacteria or in designing gene therapies.
Some promoters are bidirectional, meaning they can start transcription in both directions from the same region, producing two different RNA molecules.