PLANT PHYSIOLOGY , Vol 107, Issue 3 791-796, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Regulation of Nitrite Reductase Activity under CO2 Limitation in the Cyanobacterium Synechococcus sp. PCC7942
I. Suzuki, T. Sugiyama and T. Omata
Department of Applied Biological Sciences, School of Agricultural Sciences, Nagoya University, Chikusa, Nagoya, 464-01 Japan
During photoautotrophic growth under CO2-limited conditions, cells of
Synechococcus sp. PCC7942 excreted into the medium about 30% of the nitrite
produced by reduction of nitrate. No nitrite was excreted under
CO2-sufficient conditions. After transfer of high-CO2-grown cells to
CO2-limited conditions, nitrite reductase activity started to decline
within 0.5 h and decreased to 50% of the initial level in 3 h, whereas
nitrate reductase activity was virtually unchanged. Nitrite started to
accumulate in the medium about 3 h after the transfer of the cells to
CO2-limited conditions and reached a concentration of >0.4 mM at 17 h.
These findings suggested that the nitrite excretion was due to an imbalance
of the activities of nitrite reductase and nitrate reductase. Since
ammonium, the product of nitrite reduction, was not detected in the medium,
it was concluded that the step of nitrite reduction limits the rate of
nitrate assimilation under CO2-limited conditions. The extent of decrease
in nitrite reductase activity under CO2-limited conditions was much larger
than that caused by rifampicin (an inhibitor of RNA synthesis) treatment
under high-CO2 conditions. Addition of CO2, in the form of sodium
bicarbonate, to the CO2-limited culture increased the nitrite reductase
activity, but rifampicin inhibited this increase. These findings suggested
the presence of a mechanism that irreversibly inactivates nitrite reductase
under CO2-limited conditions.
