The utilization of selenocysteine-tRNA [Ser]Sec isoforms is regulated in part at the level of translation in vitro .

The tRNA for the 21st proteinogenic amino acid, selenocysteine, exists in mammalian cells as 2 isoforms differing by a single 2'-O-methylribosyl moiety at position 34 (Um34). These isoforms contain either 5-methoxycarbonylmethyluridine (mcm5 U) or 5-methoxycarbonylmethyl-2'-O-methyluridine (mcm5 Um) at position 34. The accumulation of the mcm5 Um isoform is tightly correlated with the expression of nonessential "stress response" selenoproteins such as glutathione peroxidase 1 (GPX1). The expression of essential selenoproteins, such as thioredoxin reductase 1 (TXNRD1), is not affected by changes in Sec-tRNA[Ser]Sec isoform accumulation. In this work we used purified mcm5 U and mcm5 Um Sec-tRNA[Ser]Sec isoforms to analyze possible differences in binding to the selenocysteine-specific elongation factor, EEFSEC, and the translation of GPX1 and TXNRD1 in vitro . Our results indicate that no major distinction between mcm5 U and mcm5 Um isoforms is made by the translation machinery, but a small consistent increase in GPX1 translation is associated with the mcm5 Um isoform. These results implicate fundamental differences in translation efficiency in playing a role in regulating selenoprotein expression as a function of isoform accumulation.