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Eukaryotic Cell doi:10.1128/EC.00204-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Regulation of Ammonia Homeostasis by the Ammonium Transporter AmtA in Dictyostelium

Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan 305-8572, Electron Microscopy Center, The Tokyo Metropolitan Institute of Medical Science, Bunkyo-ku, Tokyo, Japan 113-8613, Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka, Japan 565-0871, Department of Biofunctional Science Faculty of Agriculture and Life Science, Hirosaki University, Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan, Department of Cell Biology, Johns Hopkins University, School of Medicine, Baltimore, MD, USA 21205

	Abstract

Ammonia has been shown to function as a morphogen at multiple steps during development of the cellular slime mold Dictyostelium discoideum; however, it is largely unknown how intracellular ammonia levels are controlled. In the Dictyostelium genome, there are five genes that encode putative ammonium transporters, amtA, amtB, amtC, rhgA, and rhgB. Here, we show that AmtA regulates ammonia homeostasis during growth and development. We found that cells lacking amtA had increased levels of ammonia/ammonium, whereas their extracellular ammonia/ammonium levels were highly decreased. These results suggest that AmtA mediates excretion of ammonium. Supporting a role for AmtA in ammonia homeostasis, AmtA mRNA is expressed throughout the life cycle, and its expression level increases during development. Importantly, AmtA-mediated ammonia homeostasis is critical for many developmental processes. amtA^- cells are more sensitive to NH₄Cl in chemotaxis towards cAMP and in formation of multicellular aggregates. Furthermore, even in the absence of exogenously added ammonia, we found that amtA^- cells produced many small fruiting bodies and viability and germination of amtA^- spores were dramatically compromised. Taken together, our data clearly demonstrate that AmtA regulates ammonia homeostasis and plays important roles in multiple developmental processes in Dictyostelium.