RAT1P AND XRN1P ARE FUNCTIONALLY INTERCHANGEABLE EXORIBONUCLEASES THAT ARE RESTRICTED TO AND REQUIRED IN THE NUCLEUS AND CYTOPLASM, RESPECTIVELY

XRN1 encodes an abundant cytoplasmic exoribonuclease, Xrn1p, responsib le for mRNA turnover in yeast. A screen for bypass suppressors of the inviability of xrn1 ski2 double mutants identified dominant alleles of RAT1, encoding an exoribonuclease homologous with Xrn1p. These RAT1 a lleles restored XRN1-like functions, including cytoplasmic RNA turnove r, wild-type sensitivity to the microtubule-destabilizing drug benomyl , and sporulation. The mutations were localized to a region of the RAT 1 gene encoding a putative bipartite nuclear localization sequence (NL S), Fusions to green fluorescent protein were used to demonstrate that wild-type Rat1p is localized to the nucleus and that the mutant allel es result in mislocalization of Rat1p to the cytoplasm, Conversely, ta rgeting Xrn1p to the nucleus by the addition of the simian virus 40 la rge-T-antigen NLS resulted in complementation of the temperature sensi tivity of a rat1-1 strain, These results indicate that Xrn1p and Rat1p are functionally interchangeable exoribonucleases that function in an d are restricted to the cytoplasm and nucleus, respectively. It is lik ely that the higher eukaryotic homologs of these proteins will functio n similarly in the cytoplasm and nucleus.