Dual functions of yeast tRNA ligase in the unfolded protein response: unconventional cytoplasmic splicing of HAC1 pre-mRNA is not sufficient to release translational attenuation.

The unfolded protein response (UPR) is an essential signal transduction to cope with protein-folding stress in the endoplasmic reticulum. In the yeast UPR, the unconventional splicing of HAC1 mRNA is a key step. Translation of HAC1 pre-mRNA (HAC1(u) mRNA) is attenuated on polysomes and restarted only after splicing upon the UPR.