The Cycas revoluta-Nostoc Symbiosis: Enzyme Activities of Nitrogen and Carbon Metabolism in the Cyanobiont

SUMMARY: A comparative study was made of enzymes involved in nitrogen and carbon metabolism in the cyanobiont directly isolated from Cycas revoluta coralloid roots, and in the cultured isolate Nostoc 7422. The symbiotic Nostoc showed high activity of glutamine synthetase and glutamate synthase, the primary ammonia-assimilating enzyme system in cyanobacteria. Ammoniaassimilating glutamate dehydrogenase (GDH) activity was undetectable, although the catabolic GDH activity was high. Both glutamate-oxaloacetic acid transaminase and malate dehydrogenase showed higher activities in the symbiotic Nostoc than in the cultured Nostoc strain. The symbiotic Nostoc did not fix CO2 in vivo although in cell-free extracts both ribulose-1,5-bisphosphate carboxylase and phosphoribulokinase activities, similar to those in the cultured strain, were present.