To date the only tRNAs containing nucleosides modified with a selenium (5-carboxymethylaminomethyl-2-selenouridine and 5-methylaminomethyl-2-selenouridine) have been found in bacteria. By using tRNA anticodon-stem-loop fragments containing S2U, Se2U, or geS2U, we found that in vitro tRNA 2-selenouridine synthase (SelU) converts S2U-RNA to Se2U-RNA in a two-step process involving S2U-RNA geranylation (with ppGe) and subsequent selenation of the resulting geS2U-RNA (with SePO ). No 'direct' S2U-RNA→Se2U-RNA replacement is observed in the presence of SelU/SePO only (without ppGe). These results suggest that the in vivo S2U→Se2U and S2U→geS2U transformations in tRNA, so far claimed to be the elementary reactions occurring independently in the same domain of the SelU enzyme, should be considered a combination of two consecutive events - geranylation (S2U→geS2U) and selenation (geS2U→Se2U).

Chimeric oligonucleotides containing phosphodiester and phosphorothioate linkages have been obtained using the solid phase synthesis. The oligonucleotide parts possessing natural internucleotide phosphate bonds were assembled using commercially available nucleoside 3'-O-(2-cyanoethyl-N,N-diisopropylamino)phosphoramidites 7 whereas the phosphorothioate segment was built using nucleoside 3'-O-(2-thio-1,3,2-oxathiaphospholanes) 3. The oxidation steps, crucial for the conversion of phosphite linkages into the phosphate moieties, were conducted using tert-butylperoxy-trimethylsilane, and this reagent was not harmful to the diester phosphorothioate linkages. When P-diastereopure nucleoside 3'-O-(2-thio-1,3,2-oxathiaphospholane) monomers were employed the resulting chimeric backbone retained the P-stereoregularity of the phosphorothioate units.