SELENOCYSTEINE INCORPORATION IN EUKARYOTES - INSIGHTS INTO MECHANISM AND EFFICIENCY FROM SEQUENCE, STRUCTURE, AND SPACING PROXIMITY STUDIESOF THE TYPE-1 DEIODINASE SECIS ELEMENT

SECIS elements are stem-loop structures located in the 3' untranslated regions (UTRs) of eukaryotic selenoprotein mRNAs that are required fo r directing cotranslational selenocysteine incorporation at UGA codons . In prokaryotes, stem-loops mediating selenocysteine incorporation ar e located immediately downstream of the UGA selenocysteine codon, in t he coding region, Previous characterization studies of the mammalian S ECIS elements of type 1 deiodinase, glutathione peroxidase, and seleno protein 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 similar to 175-230-nt segments of the 3'UTRs of the selenoprotein mRNAs. Here we define the minimal function al rat type 1 deiodinase SECIS element, a 45-nt segment, the 5' bounda ry of which corresponds precisely to the 5'-most critical conserved nu cleotide identified previously. We also define base pairing requiremen ts in the stem of this element. In view of the presence of SECIS eleme nts 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 fun ction, Finally, we define a minimal spacing requirement of 51-111 nt b etween a eukaryotic UGA selenocysteine codon and SECIS element.