Evidence of G-A/A-G pair, not A-G/G-A pair, in SECIS element

The decoding of selenocysteine (Sec) codon for selenoprotein translation in animals requires the SECIS (selenocysteine insertion sequence) element in the 3'-untranslated region (3'-UTR) of selenoprotein mRNA. It has been hypothesized that SECIS has G-A/A-G pairs, which are non-Watson-Crick base pairs, and are one of the most indispensable parts for Sec translation. However, DNA structure research has shown that the DNA loop structure was closed by a G-A pair (1), although there is little evidence for G-A stability in SECIS. Our aim was to investigate the existence of a G-A/A-G base pair by electrophoretic mobility and Tm measurement, and we found that G-A/A-G bases formed a stable pair, but A-G/G-A bases did not.