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

摘要

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, 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 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 5′-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 5′-A0 fragments were observed in mutants defective in the nucleocytoplasmic transport, indicating that Gsp1p regulates rRNA processing through Dis3p, independent of nucleocytoplasmic transport.