Promoter

A promoter is the region of DNA that marks where transcription of a gene should start and in which direction it should run. It usually lies just upstream of the transcription start site. RNA polymerase, alone or with helper proteins, recognizes short conserved sequences in the promoter, binds there, opens up the double helix and begins making RNA.

In bacteria, a sigma factor guides RNA polymerase to two core elements about 10 and 35 bases upstream of the start site, with consensus sequences TATAAT and TTGACA; the first is often called the Pribnow box. Eukaryotic promoters are larger and more varied. Many genes copied by RNA polymerase II carry a TATA box roughly 25 to 35 bases upstream, which is bound by TATA-binding protein as part of a set of general transcription factors that must assemble before the polymerase can start. Enhancers and silencers, which can lie far away, then raise or lower activity.

The promoter sequence largely sets how often a gene is transcribed, so a mutation in a promoter can cause disease without changing any protein. In the lab and in synthetic biology, promoters are treated as parts: constitutive promoters stay on, while inducible ones, such as the lac promoter switched on by IPTG, let researchers turn a gene on when needed.