Controlling Element

 Controlling element is the term used by Barbara McClintock (1) to describe the new genetic elements she found in the late 1940s that “can modify and control the action of the genes themselves” and “may move from location to location within the chromosome complement without losing its identity.” We now know such elements as transposable elements—that is, discrete pieces of DNA that can move from place to place within a genome. Such elements are extremely widespread and have been found in virtually all organisms that have been examined. It is interesting that she discovered these elements that we now know to be mobile DNA before the structure of DNA was established in the early 1950s. The first molecular characterization of transposable elements came nearly 20 years later, after their discovery in bacteria, where DNAs containing them could be isolated and characterized (2.(

McClintock was a maize geneticist and cytogeneticist. Her discovery of controlling elements resulted from the study of irregular patterns of pigment in maize kernels (ie, variegations of pigment patterns) that arose in some ears. Such variegation results in changes in pigment gene expression during kernel development. We now know that such variegation reflects the insertion of a transposable element to inactivate a gene and the subsequent excision of the element at a later time, restoring gene function. Having earlier studied chromosome breaks that resulted from unusual chromosome fusions, she was also quick to appreciate that some variegation resulted from chromosomal breakage at a particular site. She proposed that there was an element dissociation (Ds( at the breakage site and suggested that variegation results from the loss of genes downstream of this site following chromosome breakage.

 Identification of an element that could cause chromosome breaks was novel, but even more dramatic was her finding that the element could move to another chromosomal site and again cause instability via chromosome breakage. McClintock realized that another element in addition to Ds was required to cause breaks at Ds and also to promote the translocation of Ds to another chromosomal position—that is, that another element encodes a product that promotes the movement of Ds. She named this other element activator (Ac) and also established that Ac could move from place to place.

We now know that Ac is an intact (autonomous) element (about 2.9 kbp) encoding a recombinase, a transposase, that acts on special sequences at the tips of the element to promote its translocation; Ds is a deleted version of Ac that lacks the transposase but still has the special terminal recombination sequences, so that Ac transposase can promote movement of Ds. Ds is called a nonautonomous element because it requires the presence of transposase provided by another element.

References

1. B. McClintock (1956) Cold Spring Harbor Symp. Quant. Biol. 21, 197–216. 

2. P. Starlinger (1977) In DNA Insertion Elements, Plasmids, and Episomes (A. I. Bukhari, J. A. Shapiro, and S. L. Adhya, eds.), Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, pp. 25–30.