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

Ammonium is a 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 the negative regulation of amt1+ by TORC1. This regulation depends on the GATA transcription factor Gaf1, whose phosphorylation is regulated by Ppe1 and PP2A phosphatases. Ppe1 is required for the nuclear accumulation of Gaf1, partly contributing to the controlled expression of amt1+. On the other hand, PP2A phosphatase is essential for the 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 amt1{Delta} amt2{Delta} tsc{Delta} mutant is more sensitive to low-ammonium conditions than the amt1{Delta} amt2{Delta} amt3{Delta} triple mutant. Thus, there may be an unidentified ammonium transporter whose plasma membrane localization is also dependent on the Tsc-Rhb1 pathway.