Telomerase Pseudoknot Needed For Catalysis

The pseudoknot found in the RNA portion of telomerase—the enzyme that adds guanine-rich segments to the ends of chromosomes—is necessary for the complex to be catalytically active, Xiaowei Zhuang, Mariana Mihalusova, and John Y. Wu of Harvard University report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1017686108). Just as it sounds, a pseudoknot is a structural feature that looks like a knot but isn’t actually one in a topological sense. By attaching two pairs of fluorescence donors and acceptors to different parts of the telomerase RNA, the Harvard team used single-molecule FRET (fluorescence resonance energy transfer) to probe the pseudoknot formation in various circumstances. When the segment making up the pseudoknot is isolated from intact RNA, the pseudoknot folds properly. However, the isolated full-length telomerase RNA misfolds, suggesting that other portions of the RNA get in the way. When the RNA binds the protein subunits, it again adopts the pseudoknot conformation. Some complexes didn’t form the pseudoknot even when bound to the proteins, but only successfully folded complexes were catalytically active.