Selective inhibition of selenocysteine tRNA maturation and selenoprotein synthesis in transgenic mice expressing isopentenyladenosine-deficient selenocysteine tRNA

Selenocysteine (Sec) tRNA (tRNA([Ser]Sec)) serves as both the site of Sec b iosynthesis and the adapter molecule for donation of this amino acid to pro tein. The consequences on selenoprotein biosynthesis of overexpressing eith er the wild type or a mutant tRNA([Ser]Sec) lacking the modified base, isop entenyladenosine, in its anticodon loop were examined by introducing multip le copies of the corresponding tRNA([Ser]Sec) genes into the mouse genome. Overexpression of wild-type tRNA([Ser]Sec) did not affect selenoprotein syn thesis. In contrast, the levels of numerous selenoproteins decreased in mic e expressing isopentenyladenosine-deficient (i(6)A(-)) tRNA([Ser]Sec) in a protein- and tissue-specific manner. Cytosolic glutathione peroxidase and m itochondrial thioredoxin reductase 3 were the most and least affected selen oproteins, while selenoprotein expression was most and least affected in th e liver and testes, respectively. The defect in selenoprotein expression oc curred at translation, since selenoprotein mRNA levels were largely unaffec ted. Analysis of the tRNA([Ser]Sec) population showed that expression of i( 6)A(-) tRNA([Ser]Sec) altered the distribution of the two major isoforms, w hereby the maturation of tRNA([Ser]Sec) by methylation of the nucleoside in the wobble position was repressed. The data suggest that the levels of i(6 )A(-) tRNA([Ser]Sec) and wild-type tRNA([Ser]Sec) are regulated independent ly and that the amount of wild-type tRNA([Ser]Sec) is determined, at least in part, by a feedback mechanism governed by the level of the tRNA([Ser]Sec ) population. This study marks the first example of transgenic mice enginee red to contain functional tRNA transgenes and suggests that i(6)A(-) tRNA([ Ser]Sec) transgenic mice will be useful in assessing the biological roles o f selenoproteins.