Rex1p deficiency leads to accumulation of precursor initiator tRNA(Met) and polyadenylation of substrate RNAs in Saccharomyces cerevisiae

A synthetic genetic array was used to identify lethal and slow-growth phenotypes produced when a mutation in TRM6, which encodes a tRNA modification enzyme subunit, was combined with the deletion of any non-essential gene in Saccharomyces cerevisiae. We found that deletion of the REX1 gene resulted in a slow-growth phenotype in the trm6-504 strain. Previously, REX1 was shown to be involved in processing the 3′ ends of 5S rRNA and the dimeric tRNA(Arg)-tRNA(Asp). In this study, we have discovered a requirement for Rex1p in processing the 3′ end of tRNA(i)(Met) precursors and show that precursor tRNA(i)(Met) accumulates in a trm6-504 rex1Δ strain. Loss of Rex1p results in polyadenylation of its substrates, including tRNA(i)(Met), suggesting that defects in 3′ end processing can activate the nuclear surveillance pathway. Finally, purified Rex1p displays Mg(2+)-dependent ribonuclease activity in vitro, and the enzyme is inactivated by mutation of two highly conserved amino acids.