Ac and Ds Elements

Insertion of transposable elements can affect gene expression in multiple ways.

In some mutant alleles created by insertion of transposable elements, the insert is in sequences 5' of the start site of transcription. In the diagrammed example, the insertion of a nonautonomous Ds element in the bronze locus of maize, black boxes represent the transcribed regions. The white box is the Ds. The lines show the non-transcribed parts of the wild type gene.
Mutant husks produce 6% of the amount of wild type transcripts.
In the presence of Ac, Ds excision events occur. Sometimes the tissue gives rise to germ cells. From such events, revertants have been recovered.
The revertants shown in the diagram also have reduced levels of transcripts in the husks, but the one with 8-bp of transposable element sequence left behind has 2-8 times the level of the other with only 6 bp left.

Many mutant alleles created by insertion of transposable elements have been studied. In some, the insertion is in exon or intron sequences. In the second diagrammed example, the insertion of a nonautonomous dSpm element in the bronze locus, black boxes are exons of the wild type Bz gene. The white box is the dSpm. The lines below show the parts of the DNA sequence that are found in mRNA in mutant and wild type.
The mutant protein has 5-10% of the enzyme activity encoded by the Bz gene.

Transposable elements are DNA sequence elements that can move into new locations in the genome.
Insertion of a transposable element at a new genome location often inactivates the gene at that location.
Reversion by excision is imperfect, leaving behind footprints of the element.
Element footprints can alter gene expression probably by altering the relative locations of promoter parts on the helical cylinder of DNA.

Transposable elements have been isolated and characterized from a variety of species (maize, snapdragon, yeast, E. coli, Drosophila, man).

E-mail inquiries to U. Melcher------------Last Updated: 24 November, 2003