text: An operon is made up of 3 basic DNA components:
  * Promoter – a nucleotide sequence that enables a gene to be transcribed. The promoter is recognized by RNA polymerase, which then initiates transcription.  In RNA synthesis, promoters indicate which genes should be used for messenger RNA creation – and, by extension, control which proteins the cell produces.
  * Operator – a segment of DNA to which a repressor binds.  It is classically defined in the lac operon as a segment between the promoter and the genes of the operon. The main operator (O1) in the lac operon is located slightly downstream of the promoter; two additional operators, O2 and O3 are located at -82 and +412, respectively.  In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes.
  * Structural genes – the genes that are co-regulated by the operon.
  Not always included within the operon, but important in its function is a regulatory gene, a constantly expressed gene which codes for repressor proteins. The regulatory gene does not need to be in, adjacent to, or even near the operon to control it.
  An inducer (small molecule) can displace a repressor (protein) from the operator site (DNA), resulting in an uninhibited operon.
  Alternatively, a corepressor can bind to the repressor to allow its binding to the operator site.  A good example of this type of regulation is seen for the trp operon.