Seryl-tRNA synthetase promotes translational readthrough by mRNA binding and involvement of the selenocysteine incorporation machinery

Translational readthrough of UGA stop codons by selenocysteine-specific tRNA (tRNASec) enables the synthesis of selenoproteins. Seryl-tRNA synthetase (SerRS) charges tRNASec with serine, which is modified into selenocysteine and delivered to the ribosome by a designated elongation factor (eEFSec in eukaryotes). Here we found that components of the human selenocysteine incorporation machinery (SerRS, tRNASec, and eEFSec) also increased translational readthrough of non-selenocysteine genes, including VEGFA, to create C-terminally extended isoforms. SerRS recognizes target mRNAs through a stem-loop structure that resembles the variable loop of its cognate tRNAs. This function of SerRS depends on both its enzymatic activity and a vertebrate-specific domain. Through eCLIP-seq, we identified additional SerRS-interacting mRNAs as potential readthrough genes. Moreover, SerRS overexpression was sufficient to reverse premature termination caused by a pathogenic nonsense mutation. Our findings expand the repertoire of selenoprotein biosynthesis machinery and suggest an avenue for therapeutic targeting of nonsense mutations using endogenous factors.