The Saccharomyces cerevisiae small GTPase, Gsp1p/Ran, is involved in 3 ' processing of 7S-to-5.8S rRNA and in degradation of the excised 5 '-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 th e relationship between the exosome and the RanGTPase cycle, a series of tem perature-sensitive Saccharomyces cerevisiae dis3 mutants were isolated and their 5.8S rRNA processing was compared with processing in strains with mut ations in a S. cerevisiae Ran homologue, Gsp1p. In both dis3 and gsp1 mutan ts, 3 ' processing of 7S-to-5.8S rRNA was blocked at three identical sites in an allele-specific manner. In contrast, the 5 ' end of 5.8S rRNA was ter minated normally in gsp1 and in did. Inhibition of 5.8S rRNA maturation in gsp1 was rescued by overexpression of nuclear exosome components Dis3p, Rrp 4p, and Mtr4p, but not by a cytoplasmic exosome component, Ski2p. Furthermo re, gsp1 and did accumulated the 5 ' -A0 fragment of 35S pre-rRNA, which is also degraded by the exosome, and the level of 27S rRNA was reduced. Neith er 5.83 rRNA intermediates nor 5 ' -A0 fragments were observed in mutants d efective in the nucleocytoplasmic transport, indicating that Gsp1p regulate s rRNA processing through Dis3p, independent of nucleocytoplasmic transport .