Transposable Elements in Maize the?Activator-Dissociation (Ac-Ds) System

Abstract

Although unstable mutants in maize (Zea mays) were described as early as 1914 (Emerson 1914, 1917, 1929; Rhoades 1936, 1938), the first explanation of such mutants in terms of transposable DNA was provided by Barbara McClintock's elegant series of experiments on the activator-dissociation (Ac-Ds) controlling-element system of maize (McClintock 1947,1948, 1951). McClintock demonstrated genetically thatAc and Ds were short regions of DNA which could move (transpose) from one chromosomal location to another. McClintock also established that Ds could transpose only in response to the action of Ac (i.e. both elements were required in the same nucleus for Ds transposition), and that Ac could transpose autonomously (i.e. in the absence of Ds). A total of eight transposable element systems have been recognized in maize, the best characterized of which are Ac(Mp)Ds, Spm and Robertson's mutator (reviewed in Fedoroff 1983; Nevers et al. 1984). All but Robertson's mutator occur as two-element systems, similar to Ac-Ds. Transposable elements have now been shown to be widespread in living organisms-occurring in prokaryotes and lower eukaryotes as well as other higher eukaryotes, including animals.