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 U4 snRNA Kink-Turn |
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U4 snRNA Kink-Turn
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Gunnar F. Schröder and Helmut Grubmüller
The kink-turn (k-turn), a new RNA structural motif found in the spliceosome and the ribosome, serves as a specific protein recognition element and as a structural building block. While the structure of the spliceosomal U4 snRNA k-turn/15.5K complex is known from a crystal structure, it is unclear whether the k-turn also exists in this folded conformation in the free U4 snRNA. Thus, we investigated the U4 snRNA k-turn by single-molecule FRET measurements in the absence and presence of the 15.5K protein and its dependence on the Na+ and Mg2+ ion concentration. We show that the unfolded U4 snRNA k-turn introduces a kink of 85 – 15 in an RNA double helix. While Na+ and Mg2+ ions induce this more open conformation of the k-turn, binding of the 15.5K protein was found to induce the tightly kinked conformation in the RNA that increases the kink to 52 – 15. By comparison of the measured FRET distances with a computer-modeled structure, we show that this strong kink is due to the kturn motif adopting its folded conformation. Thus, in the free U4 snRNA, the k-turn exists only in an unfolded conformation, and its folding is induced by binding of the 15.5K protein.
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References
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A. K. Wozniak, S. Nottrott, E. Kühn-Hölsken, G. F. Schröder, H. Grubmüller, R. Lührmann, C. A. M. Seidel, and F. Oesterhelt. Detecting protein-induced folding of the U4 snRNA kink-turn by single-molecule multiparameter FRET measurements. RNA 11: 1545-1554 (2005).
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