Area of DNA that regulates gene expression
The area of DNA ahead of each gene contains regulatory information about how and when the gene is to be expressed. The area binds to proteins (RNA polymerase) that are needed for gene expression (transcription). All genes must have a promoter in order to be expressed.
Genes transferred by genetic engineering must be accompanied by a promoter.
Different promoters can give genes different expression patterns. Some promoters are active in all cells at all times, while others are specific to different organisms or tissue types. Others are sensitive to external signals such as temperature or the presence of a certain chemical. Such promoters can be used as controllable on/off switches for genes.