Genes do not switch on by themselves. A promoter is a stretch of DNA just upstream of a gene where RNA polymerase and transcription factors assemble to begin transcription. Its sequence determines how strongly and under what conditions the gene is expressed.

Core promoter elements such as the TATA box help position the polymerase. Enhancers and silencers farther away can loop in and modulate activity. In bacteria, promoters are often shorter and more uniform; eukaryotic promoters are longer and more complex, integrating many regulatory signals.

Mutations in promoter regions can sharply reduce or increase gene expression and cause disease. Researchers insert promoters of known strength into engineered constructs to control when and where a gene is active. Mapping promoter architecture remains a central task in understanding gene regulation.

• DNA region that initiates gene transcription
• Binds RNA polymerase and transcription factors
• Controls strength and timing of expression
• Varies in complexity between bacteria and eukaryotes