S-Geranyl-2-thiouridine wobble nucleosides of bacterial tRNAs; chemical and enzymatic synthesis of S-geranylated-RNAs and their physicochemical characterization

Recently, highly lipophilic S-geranylated derivatives of 5-methylaminomethyl-2-thiouridine (mnm5geS2U) and 5-carboxymethylaminomethyl-2-thiouridine (cmnm5geS2U) were found at the first (wobble) anticodon position in bacterial tRNAs specific for Lys, Glu and Gln. The function and cellular biogenesis of these unique tRNAs remain poorly understood. Here, we present one direct and two post-synthetic chemical routes for preparing model geS2U-RNAs. Our experimental data demonstrate that geS2U-RNAs are more lipophilic than their parent S2U-RNAs as well as non-modified U-RNAs. Thermodynamic studies revealed that the S-geranyl-2-thiouridine-containing RNA has higher affinity toward complementary RNA strand with G opposite the modified unit than with A. Recombinant tRNA selenouridine synthase (SelU) exhibits sulfur-specific geranylation activity toward model S2U-RNA, which is composed of the anticodon-stem-loop (ASL) from the human tRNA(Lys3) sequence. In addition, the presence of magnesium ions is required to achieve appreciable geranylation efficiencies.