What is a promoter and what is its function?

A promoter is a DNA sequence located upstream (5') of a gene that serves as the binding site for RNA polymerase and transcription factors to initiate transcription. In prokaryotes, common promoter elements include the -10 box (TATAAT, Pribnow box) and -35 box recognized by sigma factor. Eukaryotic promoters contain elements like the TATA box, initiator element, and various regulatory sequences. The strength of a promoter determines the frequency of transcription initiation and thus gene expression levels.