An operon is a group of genes that is read together as a single unit. Operons are usually under the control of a single gene region that directs reading, called a promoter. Depending on the organism, one or more proteins assemble at the promoter to stimulate reading of the genes. The genes are often read, or transcribed, as a single long transcript, but this is not always the case. Only simple organisms, like bacteria, contain operons, and those genes are each single long stretches of DNA specialized for a particular task.
Control of an operon is tightly regulated in bacteria. Just as there are proteins that turn operons on, there are proteins that can turn the operon off. When turned on, operons are said to be induced, and when turned off, they are said to be repressed. In general, interaction with proteins turns them on, and dissociation from the proteins turns them off, but the reverse can also be true, depending on the operons.
Proteins that turn on an operon are called activators, while those that turn off operons are called inhibitors. At any given time, both activators and inhibitors are available in the cell. Most activators and inhibitors work by stimulating or blocking the promoter. Some operons are under more complex control than this, relying also on activators or inhibitors that interact with DNA far away from them. These far-off molecules are often called effectors or allosteric molecules.
The combination of simple structure and tight control has caused operons to be widespread among bacteria. An operon is an efficient evolutionary motif that works in a variety of environments. While the immediate environment of operons can at times impact their productivity, particularly if temperature or pH in a cell is not at an optimal level, the general structure of them always remains the same. Genes inside operons are never decoupled from each other, and regardless of the stress on a cell, they are always transcribed together and in their entirety.
A variety of operons are well characterized in many species of bacteria, and they have diverse functions among the different organisms. There are operons that handle production of amino acids, which are the building blocks of proteins, and operons that handle metabolism of sugars. They are extremely versatile genetic elements, allowing genes that do significantly different things to work together in concert.