Functionality of mutations at conserved nucleotides in eukaryotic SECIS elements is determined by the identity of a single nonconserved nucleotide.

In eukaryotes, the specific cotranslational insertion of selenocysteine at UGA codons requires the presence of a secondary structural motif in the 3' untranslated region of the selenoprotein mRNA. This selenocysteine insertion sequence (SECIS) element is predicted to form a hairpin and contains three regions of sequence invariance that are thought to interact with a specific protein or proteins. Specificity of RNA-binding protein recognition of cognate RNAs is usually characterized by the ability of the protein to recognize and distinguish between a consensus binding site and sequences containing mutations to highly conserved positions in the consensus sequence. Using a functional assay for the ability of wild-type and mutant SECIS elements to direct cotranslational selenocysteine incorporation, we have investigated the relative contributions of individual invariant nucleotides to SECIS element function. We report the novel finding that, for this consensus RNA motif, mutations at the invariant nucleotides are tolerated to different degrees in different elements, depending on the identity of a single nonconserved nucleotide. Further, we demonstrate that the sequences adjacent to the minimal element, although not required for function, can affect function through their propensity to base pair. These findings shed light on the specific structure these conserved sequences may form within the element. This information is crucial to the design of strategies for the identification of SECIS-binding proteins, and hence the elucidation of the mechanism of selenocysteine incorporation in eukaryotes.