Selenocysteine codons decrease polysome association on endogenous selenoprotein mRNAs

Background: Selenocysteine incorporation has been reported to be inefficien t in all systems studied, including Escherichia coli, baculovirus-insect ce ll, systems, rabbit reticulocyte in vitro translation systems, transiently transfected mammalian cells, and intact animals. Nonetheless, full-length s elenoproteins containing up to 17 selenocysteine residues are produced in a nimals, indicating that the efficiency observed in manipulated systems migh t not accurately reflect the true efficiency of this process in nature. Results: To begin to address this apparent discrepancy, we have examined th e polysome profiles of endogenously expressed selenoprotein mRNAs in a mamm alian cell line, and compared them with nonselenoprotein mRNAs, We report t hat three selenoprotein mRNAs, type 1 deiodinase, glutathione peroxidase an d selenoprotein P, are under-loaded with ribosomes, based on their predicte d open reading frame sizes, The average numbers of ribosomes per mRNA corre spond to the sizes predicted by termination at the UGA selenocysteine codon s, Appropriate loading on the type 1 deiodinase mRNA is seen following subs titution of a cysteine codon for the selenocysteine codon, indicating that the UGA codon confers a translational penalty on the mRNA, Surprisingly, ri bosomal loading is also increased by the expression of eukaryotic release f actors eRF1 and eRF3, Conclusions: These results suggest that the presence of a selenocysteine co don confers a translational penalty on selenoprotein mRNAs, and that increa sed levels of release factors may alter the kinetics of termination.