A promoter is the main regulatory portion of a gene. The simplest analogy is that a promoter is a “switch” that turns a gene “on” or “off.” It is the portion of the gene where cellular machinery binds before transcribing the DNA blueprint into a useful RNA. There are different types of RNA that may be transcribed, including messenger RNA’s (mRNAs) that encode useful proteins and regulatory RNAs that mediate gene silencing. But, the first step is always binding of an RNA polymerase to the gene’s promoter. No promoter, no useful RNAs or proteins!
In making transgenic plants, the gene of interest inserted into a plant genome must include a promoter to allow this gene to be switched “on” or transcribed. One of the most common promoters used in engineering GM crops is the CaMV (Cauliflower Mosaic Virus) or “35S”promoter, which is a type of constitutive promoter (always “on”). Different types of promoters, including those that require specific environmental signals in order to switch “on”, are found throughout the natural world.
Recent Forbes article by David Kroll on the safety of the CaMV 35S promoter: