"Controlling elements" was the term Barbara McClintock gave to transposable elements when she discovered them in maize in the late 1940s. This designation emphasized that these new genetic elements could control gene expression, as well as move from place to place in the genome (1). McClintock discovered two classes of controlling elements, one of which she designated Dissociation (Ds) and the other activator (Ac). She found that while Ds elements could move in the presence of Ac, they were unable to move alone, while Ac was capable of movement in the presence or absence of Ds. She hypothesized that activator made a product that could promote the movement of both Ac and Ds. We now know that this product is the Ac transposase and that Ds is a deletion derivative of Ac that contains the same special recombination sequences at its tips that the Ac transposase can act on, but is lacking a transposase. Because Ds lacks a transposase gene, it cannot promote its own movement and is dependent on this product from Ac. Ds is said to be a nonautonomous controlling element because it can’t move in the absence of Ac, whereas Ac is said to be an autonomous controlling element because it encodes its own transposase that can act upon its terminal sequences to move the Ac element from place to place.
Nonautonomous elements are deleted versions of autonomous elements. Different nonautonomous versions of the same element result from different internal deletions. These deleted versions probably arise from incomplete repair events at the donor site after transposon excision (2-4); this repair involves transferring information from an intact homologue, using homologous recombination and gap repair.
In addition to Ac-Ds and several other sets of autonomous and nonautonomous elements in maize, autonomous and nonautonomous elements have been also observed in other organisms such as Drosophila (2), and Caenorhabditis elegans (3, 5). Virtually all bacterial elements that have been observed appear to be autonomous.
