Selenium is biologically active through the functions of selenoproteins that 
contain the amino acid selenocysteine. This amino acid is translated in response 
to in-frame UGA codons in mRNAs that include a SECIS element in its 3' 
untranslated region, and this process requires a unique tRNA, referred to as 
tRNA([Ser]Sec). The translation of UGA as selenocysteine, rather than its use as 
a termination signal, is a candidate restriction point for the regulation of 
selenoprotein synthesis by selenium. A specialized reporter construct was used 
that permits the evaluation of SECIS-directed UGA translation to examine 
mechanisms of the regulation of selenoprotein translation. Using SECIS elements 
from five different selenoprotein mRNAs, UGA translation was quantified in 
response to selenium supplementation and alterations in tRNA([Ser]Sec) levels 
and isoform distributions. Although each of the evaluated SECIS elements 
exhibited differences in their baseline activities, each was stimulated to a 
similar extent by increased selenium or tRNA([Ser]Sec) levels and was inhibited 
by diminished levels of the methylated isoform of tRNA([Ser]Sec) achieved using 
a dominant-negative acting mutant tRNA([Ser]Sec). tRNA([Ser]Sec) was found to be 
limiting for UGA translation under conditions of high selenoprotein mRNA in both 
a transient reporter assay and in cells with elevated GPx-1 mRNA. This and data 
indicating increased amounts of the methylated isoform of tRNA([Ser]Sec) during 
selenoprotein translation indicate that it is this isoform that is 
translationally active and that selenium-induced tRNA methylation is a mechanism 
of regulation of the synthesis of selenoproteins.