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. Sargent, Grenoble) for assistance in data collection, V. Ramakrishnan for comments on the manuscript, and D. Goven for help in the footprint assays. P.W.H. is supported by an EMBO fellowship, This work was supported by the Swiss National Science Foundation (SNSF), the NCCR Structural Biology programme of the SNSF, and a Young Investigator grant from the Human Frontier Science Program