Escherichia coli tRNA 2-selenouridine synthase (SeIU) converts S2U-RNA to Se2U-RNA via S-geranylated-intermediate

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 SePO33-). No 'direct' S2U-RNA -> Se2U-RNA replacement is observed in the presence of SelU/SePO33- 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).