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1Department of Biological Sciences, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4
2School of Biological Sciences, The Flinders University of South Australia, Bedford Park, GPO Box 2100, Adelaide 5001, Australia
Author for correspondence: David A. Cotter. Tel: +1 519 253 4232 ext 5042. Fax: +1 519 971 3609. e-mail: cotter1@server.uwindsor.ca
ABSTRACT
The sori of Dictyostelium discoideum (strains SG1, SG2, NC4 and V12) contained more than 100 mM ammonium phosphate. Glutamine synthetase (GS), which could remove ammonia from the sorus, was not present in 2-d-old dormant spores but enzyme activity returned to vegetative levels after spore germination. Based on mRNA blotting, the activity of this enzyme in germinating spores appeared to be transcriptionally controlled. At the same time that GS activity was increasing, ammonia was released from germinating spores. Exogenous ammonium ions at a concentration of 28 mM did not block germination nor modulate GS activity in nascent amoebae. It was concluded that the transcription and translation of GS is not environmentally regulated but is an integral part of the germination process, preparing nascent amoebae for vegetative growth. An exogenous concentration of 69 mM ammonium phosphate could maintain dormancy in spores of strains SG1 and SG2 for at least a week in the absence of any other inhibitory component from the sori. The inhibition was reversible at any time either by dilution or by washing the spores free of the ammonium ion. Spores of strain acg- were not inhibited by 100 mM ammonium phosphate. A model is presented in which GS in prespore cells serves as a sink for ammonia to allow the osmotically sensitive adenylyl cyclase aggregation protein (ACA) to activate protein kinase A (PKA) to induce fruiting-body formation. After fruiting-body formation is complete, the decline in GS and ACA activities in developing spores is offset by their replacement with the osmotically and ammonia-stimulated adenylyl cyclase osmosensor for germination (ACG). Ammonia and discadenine may act as separate signals to synergistically activate PKA by stimulating ACG activity while inhibiting cAMP phosphodiestrase activity in fully dormant spores.
Abbreviations: ACA, adenylyl cyclase for cellular aggregation; ACG, adenylyl cyclase osmosensor for regulation of spore germination; GDH, glutamate dehydrogenase; GS, glutamine synthetase; MSX, L-methionine sulphoximine; PKA, protein kinase A;
Received 22 July 1998;
revised 8 April 1999;
accepted 20 April 1999.
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