Synthesis and characterization of modified nucleotides in the 970 hairpin loop of Escherichia coli 16S ribosomal RNA.

The synthesis of the 6-O-DPC-2-N-methylguanosine (m(2)G) nucleoside and the corresponding 5'-O-DMT-2'-O-TOM-protected 6-O-DPC-2-N-methylguanosine phosphoramidite is reported [DPC, diphenyl carbamoyl; DMT, 4,4'-dimethoxytrityl; TOM, [(triisopropylsilyl)oxy]methyl]. The availability of the phosphoramidite allows for syntheses of hairpin RNAs with site-selective incorporation of 2-N-methylguanosine modification. Four 18-nt hairpin RNA analogues representing the 970-loop region (helix 31 or h31; U960-A975) of Escherichia coli 16S rRNA were synthesized with and without modifications in the loop region. Subsequently, stabilities and conformations of the singly and doubly modified RNAs were examined and compared with the corresponding unmodified RNA. Thermodynamic parameters and circular dichroism spectra are presented for the four helix 31 RNA analogues. Surprisingly, methylations in the loop region of helix 31 slightly destabilize the hairpin, which may have subtle effects on ribosome function. The hairpin construct is suitable for future ligand-binding experiments.