Multiple ammonium transporters in fission yeast are coordinated by transcriptional and localization regulation in response to nitrogen starvation

Ammonium is the preferred nitrogen source for microorganisms. We investigated the regulation of ammonium transporters Amt1, Amt2 and Amt3 in the fission yeast Schizosaccharomyces pombe. Expression of the amt1+ gene increases under nitrogen starvation as well as in the TORC1-deficient mutant tor2-287, suggesting negative regulation of amt1+ by TORC1. This regulation depends on the GATA transcription factor Gaf1, the phosphorylation of which is regulated by Ppe1 and PP2A phosphatases. Ppe1 is required for the nuclear accumulation of Gaf1, partly contributing to the controlled expression of amt1+. In contrast, PP2A phosphatase is essential for amt1+ induction upon starvation, indicating distinct roles for Ppe1 and PP2A in Gaf1 regulation. Nitrogen starvation also promotes the plasma membrane translocation of Amt1 and Amt2 in a manner dependent on the Tsc-Rhb1 pathway. Intriguingly, the amt1Δ amt2Δ tscΔ mutant is more sensitive to low-ammonium conditions than the amt1Δ amt2Δ amt3Δ triple mutant. Thus, there might be an unidentified ammonium transporter that is also dependent on the Tsc-Rhb1 pathway for its plasma membrane localization.