SECIS elements are stem-loop structures located in the 3' untranslated regions 
(UTRs) of eukaryotic selenoprotein mRNAs that are required for directing 
cotranslational selenocysteine incorporation at UGA codons. In prokaryotes, 
stem-loops mediating selenocysteine incorporation are located immediately 
downstream of the UGA selenocysteine codon, in the coding region. Previous 
characterization studies of the mammalian SECIS elements of type 1 deiodinase, 
glutathione peroxidase, and selenoprotein P showed that conserved nucleotides in 
the loops and unpaired bulges, and base pairing in the stems are required for 
SECIS function. These initial studies utilized approximately 175-230-nt segments 
of the 3'UTRs of the selenoprotein mRNAs. Here we define the minimal functional 
rat type 1 deiodinase SECIS element, a 45-nt segment, the 5' boundary of which 
corresponds precisely to the 5'-most critical conserved nucleotide identified 
previously. We also define base pairing requirements in the stem of this 
element. In view of the presence of SECIS elements in the open reading frames 
(ORFs) of bacterial selenoproteins, we examine the effects in the type 1 
deiodinase of extending the ORF into the SECIS element, and find that this 
dramatically inhibits SECIS function. Finally, we define a minimal spacing 
requirement of 51-111 nt between a eukaryotic UGA selenocysteine codon and SECIS 
element.