promoter

A promoter is the launch pad for reading a gene. Just before the part of the DNA that gets copied into RNA sits a short stretch of sequence that says, in effect, “start transcribing here.” RNA polymerase, the enzyme that makes RNA, docks at the promoter before it begins its run.

Promoters contain recognisable signals. In bacteria these often include sequences roughly ten and thirty-five bases upstream of the start site. In eukaryotes a core promoter may carry a TATA box and other elements, and general transcription factors must assemble there to position the polymerase. The exact sequence influences how strong the promoter is — how readily polymerase begins.

A promoter sets the basal, default starting point, but it rarely acts alone. Enhancers, repressors, and activators tune how much transcription actually happens from a given promoter, so two genes with similar promoters can be expressed at very different levels depending on the regulatory company they keep.