Selenocysteine, a selenium-containing analog of cysteine, is found in the 
prokaryotic and eukaryotic kingdoms in active sites of enzymes involved in 
oxidation-reduction reactions. Its biosynthesis and cotranslational insertion 
into selenoproteins is performed by an outstanding mechanism, implying the 
participation of several gene products. The tRNA(Sec) is one of these. In 
eukaryotes, its transcription mode by RNA polymerase III differs from that of 
classical tRNA genes, both at the level of the promoter elements and 
transcription factors involved. In addition, enhanced transcription is afforded 
by a newly characterized zinc finger activator. Not only transcription of the 
gene, but also the tRNA(Sec) itself is atypical since its 2D and 3D structures 
exhibit features which set it apart from classical tRNAs. Decoding of eukaryotic 
selenocysteine UGA codons requires a stem-loop structure in the 3'UTR of mRNAs, 
the selenocysteine insertion sequence (SECIS) element. Structure probing and 
sequence comparisons led us to propose a 2D structure model for the SECIS 
element, containing a novel RNA motif composed of four consecutive 
non-Watson-Crick base-pairs. A 3D model, rationalizing the accessibility data, 
was elaborated by computer modeling. It yields indicative or suggestive evidence 
for the role that could play some conserved residues and/or structural features 
in SECIS function. These might act as signals for interaction with SBP, the 
SECIS binding protein that we have characterized.