Cell Lysis in S. pombe ura4 Mutants Is Suppressed by Loss of Functional Pub1, Which Regulates the Uracil Transporter Fur4

Schizosaccharomyces pombe Δura4 cells lyse when grown on YPD medium. A S. pombe non-essential gene deletion library was screened to determine suppressors of the lysis phenotype. Deletion of the pub1 gene, which encoded E3 ubiquitin ligase, strongly suppressed cell lysis in Δura4 cells. The Δpub1 cells displayed high sensitivity to 5-fluorouracil, a toxic analog of uracil, and this sensitivity was suppressed by deletion of fur4, which encoded a uracil transporter. Fur4 localized primarily to the Golgi apparatus and vacuoles in wild-type cells, but localization was predominantly at the plasma membrane in Δpub1 cells. Fur4 was necessary for the utilization of extracellular uracil, cytosine, or UMP. Uracil uptake activity increased in the Δpub1 strain in a Fur4-dependent manner. In addition, uracil starvation was critical for induction of cell lysis of Δura4 strains and uracil supplementation suppressed lysis. In summary, the increased uracil uptake ability of Δpub1 cells, where Fur4 was predominantly localized to the plasma membrane, resulted in suppression of cell lysis in the Δura4 background.