1. J. H. Cate, K. Zhou, and J. A. Doudna are in the Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA;
2. A. R. Gooding, E. Podell, B. L. Golden, C. E. Kundrot, and T. R. Cech are in the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA;
3. A. R. Gooding, B. L. Golden, E. Podell, and T. Cech are also with the Howard Hughes Medical Institute at Boulder, CO 80309, USA.