Absence of the glutamine-synthetase-linked methylammonium (ammonium)-transport system in the cyanobiont of Cycas-cyanobacterial symbiosis.

Using the ammonium analogue (14)CH3NH 3 (+) , ammonium transport was studied in the cyanobiont cells freshly isolated from the root nodules of Cycas revoluta. An L-methionine-DL-sulphoximine (MSX)-insensitive ammonium-transport system, which was dependent on membrane potential (ΔΨ), was found in the cyanobiont. However, the cyanobiont was incapable of metabolizing exogenous (14)CH3NH 3 (+) or NH 4 (+) because of the absence of another ammonium-transport system responsible for the uptake of ammonium for assimilation via glutamine synthetase (EC 6.3.1.2). Such a modification seems to be the result of symbiosis because the free-living cultured isolate, Anabaena cycadeae, has been shown to possess both the ammonium-transport systems.