1 Department of Chemistry and Biochemistry and The Center for the Molecular Biology of RNA, University of California at Santa Cruz, Santa Cruz, CA 95064, USA. wgscott@ucsc.edu kn@mrc-lmb.cam.ac.uk.
2 MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK. wgscott@ucsc.edu kn@mrc-lmb.cam.ac.uk.