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Sen1p Performs Two Genetically Separable Functions in Transcription and Processing of U5 Small Nuclear RNA in Saccharomyces cerevisiae

The Saccharomyces cerevisiae SEN1 gene codes for a nuclear-localized superfamily I helicase. SEN1 is an ortholog of human SETX (senataxin), which has been implicated in the neurological disorders ataxia-ocular apraxia type 2 and juvenile amyotrophic lateral sclerosis. Pleiotropic phenotypes conferred by sen1 mutations suggest that Sen1p affects multiple steps in gene expression. Sen1p is embedded in a protein–protein interaction network involving direct binding to multiple partners. To test whether the interactions occur independently or in a dependent sequence, we examined interactions with the RNA polymerase II subunit Rpb1p, which is required for transcription, and Rnt1p, which is required for 3′-end maturation of many noncoding RNAs. Mutations were identified that impair one of the two interactions without impairing the other interaction. The effects of the mutants on the synthesis of U5 small nuclear RNA were analyzed. Two defects were observed, one in transcription termination and one in 3′-end maturation. Impairment of the Sen1p–Rpb1p interaction resulted in a termination defect. Impairment of the Sen1p–Rnt1p interaction resulted in a processing defect. The results suggest that the Sen1p–Rpb1p and Sen1p–Rnt1p interactions occur independently of each other and serve genetically separable purposes in targeting Sen1p to function in two temporally overlapping steps in gene expression.