Selenium (Se) is an essential trace element primarily found in selenoproteins as 
the 21st amino acid (selenocysteine, Sec, or U). Selenoproteins play an 
important role in growth and proliferation and are typically involved in 
cellular redox balance. Selenocysteine is encoded by an in-frame UGA codon 
specified by a stem-loop structure, the Sec insertion sequence element (SECIS), 
which, in eukaryotes, is located in the 3'-untranslated region (UTR). The 
availability of the Naegleria gruberi (ATCC 30224) genome sequence and the use 
of this organism as a model system for the pathogenic amoeba N. fowleri allowed 
us to investigate the Sec incorporation pathway in this primitive eukaryote. 
Using bioinformatics tools, we identified gene sequences encoding PSTK 
(O-phosphoseryl-tRNA(Sec) kinase), SepSecS 
(O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase), SelD/SPS2 (selenophosphate 
synthetase), EFSec (selenocysteine-specific elongation factor) and SBP (SECIS 
binding protein). These findings were confirmed by RT-PCR and by sequencing. A 
potential tRNA(Ser)Sec (SelC) gene and a putative selenoprotein with sequence 
similarity to a mitochondrial thioredoxin reductase (TR3) were also identified. 
Our results show that the selenocysteine incorporation machinery is indeed 
present in N. gruberi. Interestingly, the SelD/SPS2 gene is 2214 bp in length 
and contains two distinct domains. The N-terminal region shows sequence 
similarity to predicted methyltransferase proteins, and the C-terminal region is 
homologous to prokaryotic SelD/SPS2. Our results suggest the possibility of 
novel selenoproteins.