Transposons: Part II

 
There are many eukaryotic transposons that resemble the simple bacterial transposons described in Transposons: Part I. The classic examples are the P-factor transposon in Drosophila melanogaster and the AC-like elements in maize.

Both of these transposons have many of the characteristics of the bacterial transposons including the presence of a transposase gene. Like the bacterial transposons described earlier, this type of transposon jumps from one location to another. The original genome site is restored when the transposon is excised.

Transposons were first discovered in plants because there are many plant transposons that are quite active (they jump a lot) and they frequently land in genes that become disrupted. The disrupted gene can cause a visible phenotype that plant breeders have taken note of.

One example is shown on the left. The top figure is a yellow (colorless) kernel of corn. The wild-type purple color is not produced because of a transposon (Spm) inserted into one of the genes for the production of the pigment anthocyanin. Unfortunately for the plant breeder, this mutant isn't stable and from time to time the kernels "revert" back to purple as shown in the lower figure. The purple color is not evenly distributed because the "reversion" only occurs in small clusters of cells.

It was Barbara McClintock who first recognized that this pattern was due to "jumping genes" back in the 1940's. She based her conclusions on work she was doing with a number of genes in corn where the genetics could not be reconciled with standard Mendelian transmission. We now know that the reversion to production of anthocyanin is due to excision of the Spm transposon that was disrupting the gene. This excision occurs spontaneously in the somatic cells during the development of the kernel. McClintock received the Nobel Prize in 1983 for the discovery of mobile genetic elements.

There are many other examples of transposon mediated mutations in plants, as well as in other eukaryotes, such as yeast and Drosophila melanogaster. Another plant pigment example was shown in Monday's Molecule #45. The picture of the patterned petunia flower is reproduced below. It is taken from University of Bern website.

The pattern of colored stripes seen in petunia flowers (left) is due to the presence of transposon Tph1. The species Petunia hybrida line W138 contains a disrupted rt locus due to the insertion of transposon dTph1 (Kroon et al. 1994). The mutation blocks production of anthrocyanin pigments and gives rise to a white flower.

During development of the flower, the Tph1 transposon excises in certain cells and pigment production is restored. The pie-shaped pattern of cells reveals that the flower grows outward from a small number of cells in the center of the primordial flower head.

The W138 line can be used to isolate additional mutants since Tph1 excises and reintegrates into other genes at an appreciable rate (van Houwelingen et al. 1998).

Plant genomes harbor many transposons since they have a huge amounts of junk DNA where transposons can hide without causing damage. In fact, much of this junk DNA may have originated from ancient transposons that acquired mutations rendering them unable to excise and jump to another site. Over time other transposons inserted themselves into the defective transposons and the amount of junk DNA grew. The recent sequencing of the genomes of several plants has revealed an abundance of sequences related to transposons. These sequences appear to be inactive.

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[Photo Credit: The pictures of the corn kernels are from Moran, Scrimgeour et al. Biochemistry 1998.]

Kroon, J., Souer, E., de Graaff, A., Xue, Y., Mol, J. and Koes, R. (1994) Cloning and structural analysis of the anthocyanin pigmentation locus Rt of Petunia hybrida: characterization of insertion sequences in two mutant alleles. Plant J. 5:69-80. [PubMed]

van Houwelingen, A., Souer, E., Spelt, K., Kloos, D., Mol, J. an Koes, R. (1998) Analysis of flower pigmentation mutants generated by random transpson mutagenesis in Petunia hybrida. Plant J. 13:39-50. [PubMed]