Operons:
a general overview

The discovery of operons is credited to Francois Jacob and Jacque Monod.  In 1960, Jacob described an operon as “un groupe de ge`nes a` expression coordonne´ee par un ope´rateur,” or , as in his 1961 paper with Monod, “the genetic unit of coordinated expression.”  The scientists were studying Escherichia coli, as the singular circular DNA provided an easier model to study in order to understand more complex systems.  Operons have since not been found in eukaryotes, and this is presumbly because the eurkaryotic system has many other factors that could be regulators of genes.  In a single celled organism, though, the existence of operons ensure that the organism is utilizing its energy resources in the most efficient fashion, and transcribing only the genes which are necessary and  useful in a particular situation. 

parts of an operon

An operon contains several structural genes, a promoter and an operator.  It can be mono or polycistronic. In polycistronic operons, all genes are transcribed onto a single mRNA.  This contains several start and stop codons using which a number of different proteins can be produced from a single transcript.  It is important to note that the operating condition of the operon signals the transcription (or lack thereof) of all the genes under the control of that operator.  An operon can be either inducible or represible, with positive or negative regulation.  An inducible operon is only activated when a certain molecule is present, while a repressible operon is continually (constitutively) on unless acted upon by a molecule that will repress it.  Induction is common in metabolic pathways that result in the catabolism of a substance and the inducer is normally the substrate for the pathway.  Meanwhile repression is common in metabolic pathways that result in the biosynthesis of a substance and the co-repressor is normally the end product of the pathway being regulated. 
The structural genes are the ones under the control of the operator.  The operator is a region on the DNA that interacts with the repressor protein coded by a regulator gene, also known as the i gene.  Adjacent to the operator site within the operon is  a Promoter to which the RNA polymerase binds.  Since there is an overlap between the promoter and operator regions, an occupied operator site prevents the binding of RNA polymerase to the promoter.  Thus, the operator has a key role in transcriptional control.  In inducible operons, the repressor protein attaches to the operator site, and prevents binding of the RNA polymerase until the operon is induced by the inducer-repressor complex.  In a repressible operon, the repressor protein is normally unable to bind to the operon, but when it is bound to the co-repressor (biosynthetic endproduct) then the complex is able to bind to the operator region, obstructing the promoter, and thus preventing RNA polymerase from binding.  Mutations in one or more of these genes may lead to constitutive "on" or "off" mutants.  These are often used in research to study the production patterns of genes and proteins, and in industry to produce necessary endproducts. 

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