Selenoproteins are an elite group of proteins containing a rare amino acid, 
selenocysteine (Sec), encoded by the codon, UGA. In eukaryotes, incorporation of 
Sec requires a Sec insertion sequence (SECIS) element, a stem-loop structure 
located in the 3'-untranslated regions of selenoprotein mRNAs. Here we report 
identification of a noncanonical form of SECIS element in Toxoplasma gondii and 
Neospora canine, single-celled apicomplexan parasites of humans and domestic 
animals. This SECIS has a GGGA sequence in the SBP2-binding site in place of 
AUGA previously considered invariant. Using a combination of computational and 
molecular techniques, we show that Toxoplasma and Neospora possess both 
canonical and noncanonical SECIS elements. The GGGA-type SECIS element supported 
Sec insertion in mammalian HEK 293 and NIH 3T3 cells and did so more efficiently 
than the natural mammalian SECIS elements tested. In addition, mammalian type I 
and type II SECIS elements mutated into the GGGA forms were functional but 
manifested decreased Sec insertion efficiency. We carried out computational 
searches for both AUGA and GGGA forms of SECIS elements in Toxoplasma and 
detected five selenoprotein genes, including one coding for a previously 
undescribed selenoprotein, designated SelQ, and two containing the GGGA form of 
the SECIS element. In contrast, the GGGA-type SECIS elements were not detected 
in mammals and nematodes. As a practical outcome of the study, we developed 
pSelExpress1, a vector for convenient expression of selenoproteins in mammalian 
cells. It contains an SBP2 gene and the most efficient tested SECIS element: an 
AUGA mutant of the GGGA-type Toxoplasma SelT structure.