LIGHT REGULATION OF GLUTAMINE-SYNTHETASE IN THE GREEN-ALGA MONORAPHIDIUM-BRAUNII

Glutamine-synthetase (GS; EC 6.3.1.2) and nitrate-reductase (NR; EC 1. 6.6.2) activity and protein levels were measured in crude extracts fro m alga Monoraphidium braunii Naegeli, strain 202-7d, cultures grown un der different illumination conditions. GS did not exhibit significant variations under continuous light. However, if cultures were grown und er light/dark cycles three peaks of GS activity and two of GS protein were detected during the course of the day. On the other hand, only on e peak of both NR activity and protein was found under these condition s. A different response was also observed when cultures growing either under light/dark cycles or continuous light were transferred to darkn ess: in the first case GS activity was strongly inactivated while GS p rotein levels did not change significantly; in the second case GS acti vity and protein decreased in parallel. The effect of darkness on GS a nd NR depended on the age of the cultures and was prevented by glucose addition. These results together with those obtained with photosynthe sis inhibitors point out the energy state of the cells as the mediator y agent for light-promoted GS regulation. Studies performed in the pre sence of methionine sulfoximine (MSX) under darkness allow to propose that the energy state of the cells plays a more important role than ni trogen source on GS regulation in Monoraphidium.