A group of distinct genes that are expressed and regulated as a unit. Each operon is a DNA sequence that is composed of a tightly linked cluster of genes. At the 5′ end of the gene cluster is a promoter, the site at which RNA polymerase binds to begin transcription. RNA polymerase moves down the operon, catalyzing the synthesis of an RNA molecule with a sequence complementary to DNA. Some operons contain additional promoters between genes in the operon. Only genes on the 3′ sides of these promoters are transcribed from these internal sites.