In order to elucidate roles of the 2'-O-methylation of pyrimidine nucleotide residues of tRNAs, conformations of 2'-O-methyluridylyl(3'----5')uridine (UmpU), 2'-O-methyluridine 3'-monophosphate (Ump), and 2'-O-methyluridine (Um) in 2H2O solution were analyzed by one- and two-dimensional proton NMR spectroscopy and compared with those of related nucleotides and nucleoside. As for UpU and UmpU, the 2'-O-methylation was found to stabilize the C3'-endo form of the 3'-nucleotidyl unit (Up-/Ump-moiety). This stabilization of the C3'-endo form is primarily due to an intraresidue effect, since the conformation of the 5'-nucleotidyl unit (-pU moiety) was only slightly affected by the 2'-O-methylation of the 3'-nucleotide unit. In fact even for Up and Ump, the 2'-O-methylation significantly stabilizes the C3'-endo form by 0.8 kcal/.mol-1. By contrast, for nucleosides (U and Um), the C3'-endo form is slightly stabilized by 0.1 kcal/.mol-1. Accordingly, the stabilization of the C3'-endo form by the 2'-O-methylation is primarily due to the steric repulsion among the 2-carbonyl group, the 2'-O-methyl group and the 3'-phosphate group in the C2'-endo form. For some tRNA species, 2-thiolation of pyrimidine residues is found in positions where the 2'-O-methylation is found for other tRNA species.(ABSTRACT TRUNCATED AT 250 WORDS)

'H-NMR analyses have been made on the conformations of Zthioribothymidine (s2T), 2-thiodeoxyribothyrnidine (s2dT), as well as ribothymidine (T) and deoxyribothymidine (dT). s2T and s2dT exclusively take the anti form rather than the syn form. The C3 ' -endo-gg form of the sugar moiety is remarkably stabilized on modification of T to s2T, but not on m~ifi~tion of dT to s2dT. The steric effects of the 2-~ioc~bonyl group and the 2'-hydroxyl group cause the rigidity of the C3'-endo-gg form of s2T. Such rigidity of s% probably contributes to the thermostability of 2-thiopyrimidine polyribonucleotides and extreme thermophile tRNAs.tRNA Extreme thermophile Thermostability NMR 2-Thiopyrimidine polyribonucleotide 2-Thioribothymidine