McClintock and the Ac/Ds Transposable Elements of Corn

Cloning Maize Ac and Ds Elements

Molecular Features of the Maize Ac/Ds System

Transposon Tagging

Cloning the Cf-9 Gene of Tomato by Transposon Tagging

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Molecular Features of the Maize Ac/Ds System

Somewhat astonishingly, McClintock was able to draw all of these conclusions by genetic analysis of appropriate corn stocks and her in depth understanding of the cytogenetics of corn. Her primary core of research was performed between 1944 and 1950 and was met with a great deal of skepticism. Today, though transposable elements are recognized as important components of many genomes and may have played important roles in evolution.

Once the tools of molecular biology became a part of research in plant genetics, one of the goals was to clone both Ac and Ds. As expected these two transposable elements appear to be related. In general, all Ac elements are identical, 4563 base pairs (bp) in length. Ds elements are Ac elements that have undergone deletions. The factor that stimulates the movement of Ac is a transposase protein encoded by the element. Deletions of Ac elements created Ds elements in which all or part of this transposase was eliminated. This lack of transposase activity accounts for the inability of Ds elements to move in the absence of Ac. The transposase that is encoded by Ac elements can move throughout the cell and excise any Ds or Ac element. Because of this ability, the Ac/Ds transposase is said to be trans-acting.

What molecular features are required for an element to move? First, the transposase must be present. This protein works by recognizing sequences that are in common to both Ac and Ds elements. All the elements contain a short inverted repeat sequence (10 bp) at each end that appear to be essential for transposition of the element. Another common feature that results from insertion is an 8-bp direct repeat that is generated on either end of the element. These sequences remain after excision and are footprints that mark were the element has been. Sometimes after reversion, the expression of the allele is changed because the direct repeats altered the properties of the final protein product. Thus, the presence of the transposable element in the allele changes the phenotype produced by the allele. Transposition of the element out of the allele, can also generate a new allele with new activities.

The following lists the molecular features of the Ac/Ds system of maize.

  1. Ac is 4563 bp in length
  2. contain essential 11-bp inverted repeats at the ends
  3. encodes a 3.5 kb mRNA that is translated into a 92 kDa transposase protein
  4. 101 N-terminal amino acids not required for transposition
  5. about 200 bp on each end of element necessary for transposition
  6. transposase binds to the hexamer AAACGG
  7. 8-bp direct repeats of target DNA are generated
  8. footprints (residual DNA sequences) are often left behind after the element is excised
  9. Ds are truncated versions of Ac; these can lack a transposase or the inverted repeat
The maize Ac/Ds system is the best studied, but is not the only known system. A second system, the En/Spm system of maize is more complex. With this system, the element encodes two protein that are products of alternative splicing of the single transcript. Features in common with the Ac/Ds system are the inverted repeat requirement (13 bp in size) and target site duplications (3 bp). These are the two most common classes of transposable elements, and examples of these types of elements have been found in other species.

Copyright © 1998. Phillip McClean