The Saccharomyces cerevisiae Small GTPase, Gsp1p/Ran, Is Involved in 39 Processing of 7S-to-5.8S rRNA and in Degradation of the Excised 59-A0 Fragment of 35S Pre-rRNA, Both of Which Are Carried Out by the Exosome

Dis3p, a subunit of the exosome, interacts directly with Ran. To clarify the relationship between the exosome and the RanGTPase cycle, a series of temperature-sensitive Saccharomyces cerevisiae dis3 mutants were isolated and their 5.8S rRNA processing was compared with processing in strains with mutations in a S. cerevisiae Ran homologue, Gsp1p. In both dis3 and gsp1 mutants, 39 processing of 7S-to-5.8S rRNA was blocked at three identical sites in an allele-specific manner. In contrast, the 59 end of 5.8S rRNA was terminated normally in gsp1 and in dis3. Inhibition of 5.8S rRNA maturation in gsp1 was rescued by overexpression of nuclear exosome components Dis3p, Rrp4p, and Mtr4p, but not by a cytoplasmic exosome component, Ski2p. Furthermore, gsp1 and dis3 accumulated the 59-A0 fragment of 35S pre-rRNA, which is also degraded by the exosome, and the level of 27S rRNA was reduced. Neither 5.8S rRNA intermediates nor 59-A0 fragments were observed in mutants defective in the nucleocytoplasmic transport, indicating that Gsp1p regulates rRNA processing through Dis3p, independent of nucleocytoplasmic trans-