The regulation of photosynthetic rate in Cyanobacterium

Abstract

In the higher plant, it is proposed that the CO2 assimilation rate under high CO2 concentration is limited by the rate of electron transport or RuBP regeneration. Cyanobacterial regulatory mechanism of photosynthesis is considered to be almost identical to that of C3 higher plant except CCM (CO2 concentration mechanism). However, the regulation mechanism for CO2 assimilation rate in photosynthetic Cyanobacterium has not been clear, yet. So, we produced the cyanobacterial transformant, which expressed high level of Chromatium vinosum RuBisCO with psbAI promoter of Synechococcus PCC7942. This RuBisCO high expression mutant, HX, has 2~5 fold higher total RuBisCO activity than wild type has. The expression level of the exogenous RuBisCO gene is unstable, and its total RuBisCO activity decreased to the level of wild type RuBisCO activity during successive culture. On the other hand, the mutant which is transformed by Chromatium vinosum RuBisCO with psbAII promoter of Synechocystis PCC6803 is stable and the total RuBisCO activity was about 2 fold higher than that of the wild type. We investigated the relationship between the photosynthesis rate and the RuBisCO activities in wild, HX and AIIX mutant of S. 7942, and found that there seemed to be a different mechanism on the regulation of photosynthesis rate under the high CO2 concentration from that of the higher plant, when the CCM may not be active. Even under the high CO2 concentration, the photosynthesis of Cyanobacterium is limited by the total RuBisCO activity until the activity increased up to 2 fold of that of wild type. Then the photosystem activity decreased as the RuBisCO activity increased. As the results, the photosynthesis ability of the transformant of which total RuBisCO activity increased over 2 fold of the wild type activity decreased even under the high CO2 concentration. A model simulation on computer was constructed to explain the phenomena with the GEPASI software.