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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Cloning Maize Ac and Ds Elements

One of the first reports of cloning of the maize Ac and Ds elements was by Federoff et al. in 1983 (Cell 35:235). The cloning of the elements was made possible by:

  • a cDNA clone of the waxy gene of maize
  • waxy mutants that contained either an Ac or Ds element
Federoff et al. first cloned the Ac element by obtaining hybridizing a corn stock (Ac wxm9 that contained the Ac element in the waxy gene with the cDNA for the waxy gene. Analysis of the resulting clones revealed that a DNA fragment that was 4.3 kb in size was found in the gene. Because the stock acted as it contained an Ac element, and the researchers knew the structure of the waxy gene, they concluded that the 4.3 kb fragment was indeed the Ac element.

Next a stock (wxm9) was screened with the same waxy cDNA clone. This time the researchers found that the insert was almost identical to the Ac element but lack 200 bp in the middle of the Ac element. The stock used in this experiment is a direct descendent of the Ac wxm9 used to clone the Ac element. Because this stock acts as a Ds and not an Ac element it was considered that a Ds element had been cloned. Furthermore, because the Ds element was a deletion mutant of the Ac element it was further proven that Ds elements were indeed mutant Ac elements.

The last cloning experiment used a separate transposon stock, wxm6. Whereas the insertions in Ac wxm9 were near the 3' end of wx gene, the insertion in wxm6 was more near the 5' end of the gene. This insertion, as classified as a Ds insertion because of its requirement of an Ac element for movement, was 2 kb in size. Further analysis revealed that this Ds element was generated by a 2.3 kb deletion internal to the native Ac element. These experiments clearly demonstrated at the molecular level several features of the Ac/Ds system. First, Ds elements are deletion mutants of Ac elements, and secondly, not all Ds elements have the same structure.

Copyright © 1998. Phillip McClean