Ammonium transporter genes in the fission yeast <i>Schizosaccharomyces pombe</i>: role in ammonium uptake and a morphological transition

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<jats:p>Ammonium is an important source of nitrogen for many microorganisms, including yeast, and its availability also has substantial effects on the nitrogen metabolism and development of yeast cells. Three ammonium transporter genes of the fission yeast <jats:italic>Schizosaccharomyces pombe</jats:italic>, named <jats:italic>amt1</jats:italic>, <jats:italic>amt2</jats:italic>, and <jats:italic>amt3</jats:italic>, were identified on the basis of amino acid sequence similarity to members of the ammonium transporter/methylammonium permease (Amt/Mep) family. A series of strains were constructed that carry all combinations of <jats:italic>amt</jats:italic> deletion (<jats:italic>amt</jats:italic>Δ) mutations, and tested for growth on low ammonium and resistance to the toxic ammonium analog methylammonium. The <jats:italic>amt1</jats:italic>Δ and <jats:italic>amt2</jats:italic>Δ single mutants had different growth defects, and the <jats:italic>amt1</jats:italic>Δ<jats:italic>amt2</jats:italic>Δ double mutant displayed a much more severe growth defect on ≤ 5 m<jats:sc>m</jats:sc> ammonium. All single mutants exhibited methylammonium resistance but to different extents: <jats:italic>amt2</jats:italic>Δ was the most resistant and <jats:italic>amt3</jats:italic>Δ was the least. These results suggest that the <jats:italic>amt</jats:italic> genes encode functional transporters with distinct uptake properties. In response to ammonium limitation, the wild‐type strain isogenic to the <jats:italic>amt</jats:italic>Δ mutants underwent filamentous growth underneath the surface of solid medium. No such filamentous invasive growth, however, was observed for the <jats:italic>amt1</jats:italic>Δ mutant, indicating that Amt1 transporter is required for ammonium limitation‐induced filamentous invasive growth.</jats:p>

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