A prokaryotic regulatory gene synthesizes a protein known as:

The repressor is the protein produced by the regulatory gene. The promoter is not a protein, but a region of the gene (or operon) where RNA polymerase binds to initiate transcription. The operator is not a protein, but a region of the promoter where the repressor binds, thus blocking the movement of RNA polymerase into the gene. The enhancer is found in eukaryotes only and is a region of DNA which binds regulatory proteins which then interact with promoter regions to affect the rate of transcription. The inducer is generally not a protein, but a metabolite that combines with the repressor to inactivate it, thus allowing RNA polymerase to bind to the promoter.