Nucleic Acids Research, 2005, Vol. 33, No. 5 1627 29. Sievers,A., Beringer,M., Rodnina,M.V. and Wolfenden,R. (2004) The ribosome as an entropy trap. Proc. Natl Acad. Sci. USA, 101, 7897–7901. 30. Weinger,J.S., Parnell,K.M., Dorner,S., Green,R. and Strobel,S.A. (2004) Substrate-assisted catalysis of peptide bond formation by the ribosome. Nature Struct. Mol. Biol., 11, 1101–1106. 31. Acharya,S., Fo¨ldesi,A. and Chattopadhyaya,J. (2003) The pKa of the internucleotidic 20-hydroxyl group in diribonucleoside (30 ! 50) monophosphates. J. Org. Chem., 68, 1906–1910. 32. Chamberlin,S.I., Merino,E.J. and Weeks,K.M. (2002) Catalysis of amide synthesis by RNA phosphordiester and hydroxyl groups. Proc. Natl Acad. Sci. USA, 99, 14688–14693. 33. Dorner,S., Panuschka,C., Schmid,W. and Barta,A. (2003) Mononucleotide derivatives as ribosomal P-site substrates reveal an important contribution of the 20-OH to activity. Nucleic Acids Res., 31, 6536–6542. 34. DeRose,V. (2002) Two decades of RNA catalysis. Chem. Biol., 9, 961–969.