Role of nuclear pools of Aminoacyl-tRNA synthetases in tRNA nuclear export

Reports of nuclear tRNA aminoacylation and its role in tRNA nuclear export (Lund and Dahlberg, 1998; Sarkar et al., 1999; Grosshans et al., 2000a) hav e led to the prediction that there should be nuclear pools of aminoacyl-tRN A synthetases. We report that in budding yeast there are nuclear pools of t yrosyl-tRNA synthetase, Tys1p. By sequence alignments we predicted a Tys1p nuclear localization sequence and showed it to be sufficient for nuclear lo cation of a passenger protein. Mutations of this nuclear localization seque nce in endogenous Tys1p reduce nuclear Tys1p pools, indicating that the mot if is also important for nucleus location. The mutations do not significant ly affect catalytic activity, but they do cause defects in export of tRNAs to the cytosol. Despite export defects, the cells are viable, indicating th at nuclear tRNA aminoacylation is not required for all tRNA nuclear export paths. Because the tRNA nuclear exportin, Los1p, is also unessential, we te sted whether tRNA aminoacylation and Los1p operate in alternative tRNA nucl ear export paths. No genetic interactions between aminoacyl-tRNA synthetase s and Los1p were detected, indicating that tRNA nuclear aminoacylation and Los1p operate in the same export pathway or there are more than two pathway s for tRNA nuclear export.