text: In genetics, a regulator gene, regulator, or regulatory gene is a gene involved in controlling the expression of one or more other genes.  Regulatory sequences, which encode regulatory genes, are often at the five prime end (5) to the start site of transcription of the gene they regulate. In addition, these sequences can also be found at the three prime end (3) to the transcription start site.  In both cases, whether the regulatory sequence occurs before (5) or after (3) the gene it regulates, the sequence is often many kilobases away from the transcription start site.  A regulator gene may encode a protein, or it may work at the level of RNA, as in the case of genes encoding microRNAs.  An example of a regulator gene is a gene that codes for a repressor protein that inhibits the activity of an operator (a gene which binds repressor proteins thus inhibiting the translation of RNA to protein via RNA polymerase).
  In prokaryotes, regulator genes often code for repressor proteins. Repressor proteins bind to operators or promoters, preventing RNA polymerase from transcribing RNA.  They are usually constantly expressed so the cell always has a supply of repressor molecules on hand.  Inducers cause repressor proteins to change shape or otherwise become unable to bind DNA, allowing RNA polymerase to continue transcription. 
  Regulator genes can be located within an operon, adjacent to it, or far away from it.
  Other regulatory genes code for activator proteins.  An activator binds to a site on the DNA molecule and causes an increase in transcription of a nearby gene.  In prokaryotes, a well-known activator protein is the catabolite activator protein (CAP), involved in positive control of the lac operon.
  In the regulation of gene expression, studied in evolutionary developmental biology (evo-devo), both activators and repressors play important roles.
  Regulatory genes can also be described as positive or negative regulators, based on the environmental conditions that surround the cell.  Positive regulators are regulatory elements that permit RNA polymerase binding to the promoter region, thus allowing transcription to occur.  In terms of the lac operon, the positive regulator would be the CRP-cAMP complex that must be bound close to the site of the start of transcription of the lac genes.  The binding of this positive regulator allows RNA polymerase to bind successfully to the promoter of the lac gene sequence which advances the transcription of lac genes; lac Z, lac Y, and lac A. Negative regulators are regulatory elements which obstruct the binding of RNA polymerase to the promoter region, thus repressing transcription.  In terms of the lac operon, the negative regulator would be the lac repressor which binds to the promoter in the same site that RNA polymerase normally binds.  The binding of the lac repressor to RNA polymerase's binding site inhibits the transcription of the lac genes.  Only when a corepressor is bound to the lac repressor will the binding site be free for RNA polymerase to carry out transcription of the lac genes.