Your Synapse or Mine? Promiscuous Protein Exchanges between Neighboring Synapses

Abstract

September 2006 | Volume 4 | Issue 9 | e304 One reason to sequence the genomes of non-human organisms is to better understand our similarities and differences. And, at fi rst sight, it is hard to imagine a eukaryote more different from humans than Tetrahymena thermophila. A relative of Paramecium, this single-celled creature has a strong but fl exible exterior covered with rows of cilia; but it is inside where things seem to get really alien. Each cell contains not one but two nuclei: a micronucleus, which contains only fi ve chromosomes, and a macronucleus, which has more than 200. Biologists have long known that the micronucleus contains the DNA reserved for reproduction, and that the macronucleus arises from the micronucleus and controls the cell’s other functions. During macronucleus formation (which happens each time the cells mate), each of the fi ve chromosomes splinters into multiple fragments, which in turn replicate to form many copies of the resulting smaller chromosomes. In a new study, Jonathan Eisen and a team of over 50 scientists report the full sequence of the macronuclear genome. The authors began by isolating DNA from purifi ed macronuclei (no mean feat in itself), and performed a “shotgun” sequence, splitting the DNA into millions of fragments, sequencing each of these, and then reconstructing the whole by using computers to match overlaps. They estimate that they have captured more than 95% of the genome, and conclude it is 105 million base pairs in length. The exact number of chromosomes is still at issue, though the authors present evidence that it lies between 185 and 287, and, based on the number of telomeres, is probably about 225. T. thermophila macronuclear chromosomes, unlike those in the micronucleus and other species, are highly unusual because they appear to lack centromeres, the regions that link chromosomal replicants and then guide their separation during mitosis and meiosis. This makes some sense, since the macronucleus undergoes neither process. Furthermore, they contain much less repetitive DNA than most other eukaryotes—about 2% of the total DNA, versus over 50% in humans—partly because most repetitive DNA is jettisoned during the formation of the macronucleus, when about 15% of micronuclear genomic DNA is excised. The authors provide evidence that excision targets not only repeated elements per se but also foreign DNA (such as “selfi sh” mobile DNA transposons) in particular, indicating the importance of this process in maintaining the integrity of the expressed genome from such outside invasions. Sequencing the genome also allowed the authors to address a nagging evolutionary question, namely the timing of plastid acquisition in the alveolates, a group of three related phyla: the ciliates (including Tetrahymena), the apicomplexans (parasites that cause malaria, among other diseases), and the dinofl agellates (ocean-dwelling photosynthetic protozoans). Plastids, such as the chloroplast, are organelles descended Ciliate Genome Sequence Reveals Unique Features of a Model Eukaryote

DOI: 10.1371/journal.pbio.0040297

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Cite this paper

@article{Chanut2006YourSO, title={Your Synapse or Mine? Promiscuous Protein Exchanges between Neighboring Synapses}, author={Françoise Chanut}, journal={PLoS Biology}, year={2006}, volume={4} }