An operon

Operon

In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...

 is a set of genes controlled by a single DNA binding protein. Multiple operons controlled by the same repressor are grouped together as a regulon

Regulon

In cell biology and genetics, a regulon is a collection of genes or operons under regulation by the same regulatory protein. This term is generally used for prokaryotic systems, for example quorum sensing in bacteria. It is a group of operons/genes spread around the chromosome but controlled by a...

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A modulon is a regulon concerned with multiple pathways or functions, in which operons may be under individual controls as well as common, pleiotropic regulatory protein.

For example: the CAP modulon contains all regulons/operons, such as the lac operon and ara regulon, that are regulated by CAP

Catabolite Activator Protein

Catabolite Activator Protein or CAP is a transcriptional activator that exists as a homodimer in solution, with each subunit comprising a ligand-binding domain at the N-terminus , which is also responsible for the dimerization of the protein, and a DNA-binding domain at the C-terminus...

/cAMP

Cyclic adenosine monophosphate

Cyclic adenosine monophosphate is a second messenger important in many biological processes...

, but each operon has other regulators as well.