Abstract
The fission yeast Schizosaccharomyces pombe undergoes a switch from yeast to filamentous invasive growth in response to certain environmental stimuli. Among them is ammonium limitation. Amt1, one of the three ammonium transporters in this yeast, is required for the ammonium limitation-induced morphological transition; however, the underlying molecular mechanism remains to be understood. Cells lacking Amt1 became capable of invasive growth upon increasing concentrations of ammonium in the medium, suggesting that the ammonium taken up into the cell or a metabolic intermediate in ammonium assimilation might serve as a signal for the ammonium limitation-induced morphological transition. To investigate the possible role of ammonium-metabolizing enzymes in the signaling process, deletion mutants were constructed for the gdh1, gdh2, gln1, and glt1 genes, which were demonstrated by enzyme assays to encode NADP-specific glutamate dehydrogenase, NAD-specific glutamate dehydrogenase, glutamine synthetase, and glutamate synthase, respectively. Growth tests on various nitrogen sources revealed that a gln1Δ mutant was a glutamine auxotroph and that a gdh1Δ mutant had a defect in growth on ammonium, particularly at high concentrations. The latter observation indicates that the NADP-specific glutamate dehydrogenase of S. pombe plays a major role in ammonium assimilation under high ammonium concentrations. Invasive growth assays showed that gdh1Δ and glt1Δ mutants underwent invasive growth to a lesser extent than did wild-type strains. Increasing the ammonium concentration in the medium suppressed the invasive growth defect of the glt1Δ mutant, but not the gdh1Δ mutant. These results suggest that the nitrogen status of the cell is important in the induction of filamentous invasive growth in S. pombe.
Highlights
Many fungi undergo a morphological transition from yeast to filamentous growth in response to changes in environmental conditions, such as ammonium limitation
The budding yeast Saccharomyces cerevisiae has three ammonium transporters, Mep1, Mep2, and Mep3 [7], and the high-affinity transporter Mep2 is required for diploid pseudohyphal growth in response to ammonium limitation [1]
When grown on LNB agar medium, which contains 0.76 mM ammonium as the nitrogen source, S. pombe forms microcolonies of filamentously growing cells underneath the surface of the medium ([4] and S4 Fig), and the presumed high-affinity transporter Amt1 is required for this ammonium limitation-induced filamentous invasive growth [4]
Summary
Many fungi undergo a morphological transition from yeast to filamentous growth in response to changes in environmental conditions, such as ammonium limitation. It has been reported for a variety of fungi [1,2,3,4,5] that the ammonium limitation-induced morphological transition requires certain members of the Amt/Mep/Rh family of ammonium transport proteins [6]. The budding yeast Saccharomyces cerevisiae has three ammonium transporters, Mep, Mep, and Mep3 [7], and the high-affinity transporter Mep is required for diploid pseudohyphal growth in response to ammonium limitation [1]. The mechanism by which Mep senses ammonium and activates the downstream pathways, remains to be elucidated
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