NH4+ ASSIMILATION AND NITROGENASE REGULATION IN THE LICHEN <em>PELTIGERA APHTHOSA</em> WILLD.

The lichen Peltigera aphthosa consists of a green alga Coccomyxa, a fungus, and a N2-fixing Nostoc which is located in superficial packets (cephalodia). In symbiosis the Nostoc shows only low activities of glutamine synthetase, but the cephalodia show very high activities of glutamate dehydrogenase. Alanine dehydrogenase activity is rather similar in all components. The Nostoc liberates over 95% of the N2 which it fixes as NH4+, and fungal glutamate dehydrogenase in the cephalodia appears to be important in the assimilation of this NH4+. Exogenous NH4+(5 mM) has no effect on acetylene reduction activity by excised cephalodia but completely inhibits acetylene reduction by cephalodia attached to lichen discs (of fungus + Coccomyxa) within 24 h. This inhibition can be overcome by inhibiting main thallus glutamine synthetase activity by adding the glutamate analogue, L-methionine-SR-sulphoximine (MSX). On adding exogenous NH4+ to main thallus discs with cephalodia, glutamine accumulates in the cephalodia when main thallus glutamine synthetase is active but not when it is inactivated by MSX. Exogenous glutamine also specifically inhibits nitrogenase activity by the cephalodia although exogenous alanine and glutamate (all 5 mM) have no such effect. The data indicate that glutamine produced in the main thallus may be involved in the regulation of nitrogenase activity by the cephalodial Nostoc. [ABSTRACT FROM AUTHOR]