Suppression of inositol pyrophosphate toxicosis and hyper-repression of the fission yeast  PHO  regulon by loss-of-function mutations in chromatin remodelers Snf22 and Sol1.
Inositol pyrophosphates are signaling molecules that regulate cellular phosphate homeostasis in eukaryal taxa. In fission yeast, where the phosphate regulon (comprising phosphate acquisition genes  pho1 ,  pho84 , and  tgp1 ) is repressed under phosphate-replete conditions by lncRNA-mediated transcriptional interference, mutations of inositol pyrophosphatases that increase IP 8  levels derepress the  PHO  regulon by eliciting precocious termination of lncRNA transcription. Asp1 pyrophosphatase mutations resulting in too much IP 8  are cytotoxic in YES medium owing to overexpression of glycerophosphodiester transporter Tgp1. IP 8  toxicosis is ameliorated by mutations in cleavage/polyadenylation and termination factors, perturbations of the Pol2 CTD code, and mutations in SPX domain proteins that act as inositol pyrophosphate sensors. Here, we show that IP 8  toxicity is alleviated by deletion of  snf22  + , the gene encoding the ATPase subunit of the SWI/SNF chromatin remodeling complex, by an ATPase-inactivating  snf22- ( D996A-E997A ) allele, and by deletion of the gene encoding SWI/SNF subunit Sol1. Deletion of  snf22 +   hyper-repressed  pho1  expression in phosphate-replete cells; suppressed the  pho1  derepression elicited by mutations in Pol2 CTD, termination factor Seb1, Asp1 pyrophosphatase, and 14-3-3 protein Rad24 (that favor precocious  prt  lncRNA termination); and delayed  pho1  induction during phosphate starvation. RNA analysis and lack of mutational synergies suggest that Snf22 is not impacting 3'-processing/termination. Using reporter assays, we find that Snf22 is important for the activity of the  tgp1  and  pho1  promoters, but not for the promoters that drive the synthesis of the  PHO -repressive lncRNAs. Transcription profiling of  snf22 ∆ and  snf22- ( D996A-E997A ) cells identified an additional set of 66 protein-coding genes that were downregulated in both mutants.IMPORTANCERepression of the fission yeast  PHO  genes  tgp1 ,  pho1 , and  pho84  by lncRNA-mediated interference is sensitive to inositol pyrophosphate dynamics. Cytotoxic  asp1-STF  alleles derepress the  PHO  genes via the action of IP 8  as an agonist of precocious lncRNA 3'-processing/termination. IP 8  toxicosis is alleviated by mutations of the Pol2 CTD and the 3'-processing/termination machinery that dampen the impact of toxic IP 8  levels on termination. In this study, a forward genetic screen revealed that IP 8  toxicity is suppressed by mutations of the Snf22 and Sol1 subunits of the SWI/SNF chromatin remodeling complex. Genetic and biochemical evidence indicates that the SWI/SNF is not affecting 3'-processing/termination or lncRNA promoter activity. Rather, SWI/SNF is critical for firing the  PHO  mRNA promoters. Our results implicate the ATP-dependent nucleosome remodeling activity of SWI/SNF as necessary to ensure full access of  PHO -activating transcription factor Pho7 to its binding sites in the  PHO  mRNA promoters.
Schwer B, Innokentev A, Sanchez AM, Garg A, Shuman S