1 Department of chemistry Yale University, 225 Prospect St, New Haven, CT 06511-8499.
2 European Molecular Biology Laboratory, European Bioinformatics Institute, Wellcome Genome Campus, Hinxton, Cambridge CB10 1SD, UK.
3 Université de Strasbourg, Institut de biologie moléculaire et cellulaire du CNRS, Architecture et Réactivité de l'ARN, Strasbourg, France; e.westhof@ibmc-cnrs.unistra.fr.
4 Department of Mathematics and Statistics Bowling Green State University Bowling Green, OH 43403.