The crystal structure of UUCG tetraloop

All large structured RNAs contain hairpin motifs made of a stem closed by s everal looped nucleotides. The most frequent loop motif is the UUCG one. Th is motif belongs to the,tetraloop family and has the peculiarity of being h ighly thermodynamically stable. Here, we report the first crystal structure of two UUCG tetraloops embedded in a larger RNA-protein complex solved at 2.8 Angstrom resolution. The two loops present in the asymmetric unit are i n a different crystal packing environment but, nevertheless, have an identi cal conformation. The observed structure is globally close to that obtained in solution by nuclear magnetic resonance. However, subtle differences poi nt to a more detailed picture of the role played by 2'-hydroxyl groups in s tabilising this tetraloop. (C) 2000 Academic Press.