Suppressor mutations that make the essential transcription factor Spn1/Iws1 dispensable in Saccharomyces cerevisiae

Spn1/Iws1 is an essential eukaryotic transcription elongation factor that is conserved from yeast to humans as an integral member of the RNA polymerase II elongation complex. Several studies have shown that Spn1 functions as a histone chaperone to control transcription, RNA splicing, genome stability, and histone modifications. However, the precise role of Spn1 is not understood, and there is little understanding of why it is essential for viability. To address these issues, we have isolated 8 suppressor mutations that bypass the essential requirement for Spn1 in Saccharomyces cerevisiae. Unexpectedly, the suppressors identify several functionally distinct complexes and activities, including the histone chaperone FACT, the histone methyltransferase Set2, the Rpd3S histone deacetylase complex, the histone acetyltransferase Rtt109, the nucleosome remodeler Chd1, and a member of the SAGA coactivator complex, Sgf73. The identification of these distinct groups suggests that there are multiple ways in which Spn1 bypass can occur, including changes in histone acetylation and alterations in other histone chaperones. Thus, Spn1 may function to overcome repressive chromatin by multiple mechanisms during transcription. Our results suggest that bypassing a subset of these functions allows viability in the absence of Spn1.