REGULATION OF THE EXPRESSION OF FERREDOXIN-NITRITE REDUCTASE IN SYNCHRONOUS CULTURES OF CHLAMYDOMONAS-REINHARDTII

The regulation of ferredoxin-nitrite reductase - the second enzyme inv olved in the nitrate assimilatory pathway - in synchronous cultures of C. reinhardtii has been studied both at the activity and protein leve ls using specific antibodies. During a cycle of 12 h light/12 h dark ( 12L:12D), ferredoxin-nitrite reductase activity shows a 24-h fluctuati on with a maximum in the middle of the light period. The increase of a ctivity during the first few hours of the light phase is due to de nov o synthesis of the enzyme. This synthesis occurs in the absence of NH: and it is highly induced by either nitrate or nitrite, but it does no t require light so long as carbon skeletons are available. The decreas e of ferredoxin-nitrite reductase activity during the last hours of th e light period and during the dark phase is suggested to be due to pro tein degradation, although this process is slow because of the high st ability of the enzyme. The changes in the level of ferredoxin-nitrite reductase seem to be related to events in the cell cycle under the ill umination conditions used. Thus, synthesis of the enzyme correlates to growth periods within the cell cycle, and it does not seem to be unde r the control of a circadian rhythm.