The 9/4 secondary structure of eukaryotic selenocysteine tRNA: More pieces of evidence

Selenocysteine biosynthesis and its cotranslational incorporation into selenoproteins are achieved by a complex molecular machinery (reviewed in Hüttenhofer & Böck, 1998). A major wheel in this mechanism is the tRNASec, which plays a pivotal role. It is first charged with serine by the conventional SerRS, the seryl-residue being further converted in situ to the selenocysteyl-residue by the selenocysteine synthase enzyme. The charged selenocysteyl-tRNASec delivers selenocysteine to the nascent polypeptide chain in response to a reprogrammed UGA codon. The classical elongation factors EF-Tu (in bacteria) or EF1-α (in eukaryotes) do not intervene at this stage. Instead, this process requires a selenocysteine-specific translation factor, called SELB in bacteria, but for which no eukaryotic homologue has been cloned yet. Interestingly, antideterminants against EF-Tu binding were found in the Escherichia coli tRNASec (Rudinger et al., 1996). Based on the several functions that this tRNA has to accomplish, it is reasonable to expect that the tRNASec secondary structure should exhibit distinctive structural features deviating from classical elongator tRNAs. In this regard, functional studies, structural probing, and sequence comparisons confirmed the earlier proposal that the bacterial tRNASec needs an 8-bp long amino acceptor stem and a 6-bp long D-stem to function, instead of the canonical 7 bp and 3/4 bp, respectively (Leinfelder et al., 1988; Baron et al., 1990, 1993; Tormay et al., 1994).