Pus1p-dependent tRNA pseudouridinylation becomes essential when tRNA biogenesis is compromised in yeast

Yeast Pus1p catalyzes the formation of pseudouridine (psi) at specific site s of several tRNAs, but its function is not essential for cell viability. W e show here that Pus1p becomes essential when another tRNA: pseudouridine s ynthase, Pus4p, or the essential minor tRNA for glutamine are mutated. Stri kingly, this mutant tRNA, which carries a mismatch in the T psiC arm, displ ays a nuclear export defect. Furthermore, nuclear export of at least one wi ld-type tRNA species becomes defective in the absence of Pus1p. Our data, t hus, show that the modifications formed by Pus1p are essential when other a spects of tRNA biogenesis or function are compromised and suggest that impa irment of nuclear tRNA export in the absence of Pus1p might contribute to t his phenotype.