The fission yeast Schizosaccharomyces pombe has two distinct tRNase Z^Ls encoded by two different genes and differentially targeted to the nucleus and mitochondria

Abstract

tRNase Z is the endonuclease that is involved in tRNA 3′-end maturation by removal of the 3′-trailer sequences from tRNA precursors. Most eukaryotes examined to date, including the budding yeast Saccharomyces cerevisiae and humans, have a single long form of tRNase Z (tRNase Z^L). In contrast, the fission yeast Schizosaccharomyces pombe contains two candidate tRNase Z^Ls encoded by the essential genes sptrz1⁺ and sptrz2⁺. In the present study, we have expressed recombinant SpTrz1p and SpTrz2p in S. pombe. Both recombinant proteins possess precursor tRNA 3′-endonucleolytic activity in vitro. SpTrz1p localizes to the nucleus and has a simian virus 40 NLS (nuclear localization signal)-like NLS at its N-terminus, which contains four consecutive arginine and lysine residues between residues 208 and 211 that are critical for the NLS function. In contrast, SpTrz2p is a mitochondrial protein with an N-terminal MTS (mitochondrial-targeting signal). High-level overexpression of sptrz1⁺ has no detectable phenotypes. In contrast, strong overexpression of sptrz2⁺ is lethal in wild-type cells and results in morphological abnormalities, including swollen and round cells, demonstrating that the correct expression level of sptrz2⁺ is critical. The present study provides evidence for partitioning of tRNase Z function between two different proteins in S. pombe, although we cannot rule out specialized functions for each protein.