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PLoS Genet. 2016 Mar 7;12(3):e1005842. doi: 10.1371/journal.pgen.1005842. eCollection 2016 Mar.

A Parallel G Quadruplex-Binding Protein Regulates the Boundaries of DNA Elimination Events of Tetrahymena thermophila.

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Department of Biology, Washington University in St. Louis, St. Louis, Missouri, United States of America.


Guanine (G)-rich DNA readily forms four-stranded quadruplexes in vitro, but evidence for their participation in genome regulation is limited. We have identified a quadruplex-binding protein, Lia3, that controls the boundaries of germline-limited, internal eliminated sequences (IESs) of Tetrahymena thermophila. Differentiation of this ciliate's somatic genome requires excision of thousands of IESs, targeted for removal by small-RNA-directed heterochromatin formation. In cells lacking LIA3 (ΔLIA3), the excision of IESs bounded by specific G-rich polypurine tracts was impaired and imprecise, whereas the removal of IESs without such controlling sequences was unaffected. We found that oligonucleotides containing these polypurine tracts formed parallel G-quadruplex structures that are specifically bound by Lia3. The discovery that Lia3 binds G-quadruplex DNA and controls the accuracy of DNA elimination at loci with specific G-tracts uncovers an unrecognized potential of quadruplex structures to regulate chromosome organization.

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