UGA CODON POSITION AFFECTS THE EFFICIENCY OF SELENOCYSTEINE INCORPORATION INTO GLUTATHIONE PEROXIDASE-1

A UGA codon and a selenocysteine insertion sequence in the 3'-untransl ated region are the only established mRNA elements necessary for selen ocysteine (Sec or U) incorporation during translation. These two eleme nts, however, do not universally confer efficient Sec incorporation, T he objective of this study was to systematically examine the effect of UGA codon position on efficiency of Sec insertion. In a glutathione p eroxidase-1 (F-GPX1) expression vector, the UGA at the native position (U47) was mutated to a cysteine codon, and codons for Ser-7, Ser-12, Ser-18, Ser-29, Ser-45, Ser-93, Cys-154, Val-172, Ser-178, and Ser-195 were individually mutated to UGA and transiently expressed in COS-7 c ells. Se-75 incorporation at the 11 positions was 31, 72, 54, 105, 90, 100, 146, 135, 13, 11, and 43%, respectively, of Se-75 incorporation at U47, suggesting that Sec is more efficiently incorporated at UGA co dons positioned in the middle of the coding region rather than close t o the 5' or 3' ends. Ribonuclease protection showed that these differe nces were not due to differences in mRNA level. When the green fluores cence protein (GFP) coding region was placed in-frame at the 5' or 3' ends of the coding region in F-GPX1 to produce chimeric 50-51-kDa GFP/ GPX1 proteins, Sec incorporation at UGA codons, formerly close to the 5' or 3' ends, was increased to levels comparable to the UGA at U47, I nsertion of GFP after the UAA-stop was just as effective in increasing Sec insertion efficiency as GFP inserted before the stop. These studi es used a recombinant expression model that incorporated Sec at non-na tive UGA codons at rates equal to those of endogenous glutathione pero xidase-1 and showed that the efficiency of Sec incorporation can be mo dulated by UGA position; Sec incorporation at high efficiency appears to require that the UGA be >21 nucleotides from the AUG-start and >204 nucleotides from the selenocysteine insertion sequence element.