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

Schizosaccharomyces pombe Δ ura4 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 Δ ura4 cells. The Δ pub1 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 Δ pub1 cells. Fur4 was necessary for the utilization of extracellular uracil, cytosine, or UMP. Uracil uptake activity increased in the Δ pub1 strain in a Fur4-dependent manner. In addition, uracil starvation was critical for induction of cell lysis of Δ ura4 strains and uracil supplementation suppressed lysis. In summary, the increased uracil uptake ability of Δ pub1 cells, where Fur4 was predominantly localized to the plasma membrane, resulted in suppression of cell lysis in the Δ ura4 background.