Molecular Biology

Domestic Tidying-Up

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Science  20 Nov 2009:
Vol. 326, Issue 5956, pp. 1043
DOI: 10.1126/science.326.5956.1043-b

Ciliates, such as Paramecium and Tetrahymena, are single-celled eukaryotes that deal with the junk DNA infesting their genomes in a truly dramatic manner. They harbor complete copies of their genome within two separate nuclei. The micronucleus (or MIC) gives rise both to future progeny and to the macronucleus (or MAC), wherein the DNA is shredded, rearranged, and amplified, which allows the essential core of the genome—in some cases as little as 5% of it—to be expressed at high levels. The repetitive parasitic sequences that have been derived from transposable elements are trashed in the process.

Baudry et al. identify the enzyme (named PiggyMac) responsible for shredding the Paramecium MAC genome, and it turns out to have arisen from the very sequences that it so ruthlessly eliminates. PiggyMac is derived from transposases, which are enzymes that normally promote the spread of selfish DNA through the genome that has been “domesticated” during ciliate evolution; although this enzyme can still excise transposon remnants from the host genome, it does not catalyze their reinsertion elsewhere, and instead the junk sequences are safely disposed of. PiggyMac is critical for Paramecium development, and the supercharged expression made feasible by abbreviating and amplifying the MAC genome probably facilitates rapid growth and division. Domestication of an enemy of the state has also occurred in our immune system, in which antibody diversification is driven by the recombination-activating gene RAG, another erstwhile transposase.

Genes Dev. 23, 2478 (2009).

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