PARENT SESSION

Symposium S5B Light, redox and metabolic regulation: Dark reactions
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 510B
Chair: Peter Schürmann
Co-Chair: Toru Hisabori

Physiological function of CP12 involved in regulation of the Calvin cycle in cyanobacteria. Masahiro Tamoi^*,1, Takashi Miyazaki¹, Tamo Fukamizo¹, Shigeru Shigeoka¹, ¹ Faculty of Agriculture, Kinki University, Nara, Japan

ABSTRACT- We focused on CP12 as a regulatory factor of the Calvin cycle in cyanobacteria that was not regulated by the ferredoxin/thioredoxin system, unlike that of higher plants. The CP12 from higher plants, eukaryotic algae, and Synechocystis PCC6803 contained four cysteine (Cys) residues essential for the formation of two peptide loops to interact with PRK and GAPDH. However, the CP12 from Synechococcus PCC7942 lacked the two Cys residues involved in the formation of the N-terminal peptide loop essential for the molecular interaction between CP12 and PRK. The concentrations of NAD(H) and NADP(H) in S. 7942 cells in dark conditions were 128 ± 2.5 M and 483 ± 4.0 M, respectively. Gel filtration and immunoblot analysis using crude extracts of S. 7942 showed the existence of a 520-kDa PRK/CP12/GAPDH complex in the presence of 128 M NAD⁺ and 483 M NADPH. Under light conditions, the concentrations of NAD(H) and NADP(H) in the cells were 100 ± 5.0 M and 649 ± 7.0 M, respectively, and the PRK/CP12/GAPDH complex was dissociated in the crude extract. These data suggest that the reversible dissociation of PRK/CP12/GAPDH complex is mediated by the change of NADP(H):NAD(H) ratio under light/dark conditions. The CP12-disrupted mutant cells showed almost the same growth as wild-type cells under continuous light conditions. However, under light/dark cycle (12 h/12 h), the growth rate of CP12-disrupted mutant cells was significantly inhibited compared with that of wild-type cells. The mutant cells showed decreased O₂ consumption rate and increased level of the ribulose 1,5-bisphosphate content compared with wild-type cells in dark conditions. These data suggest that the oligomerization of CP12 together with PRK and NADP⁺-GAPDH regulates the activities of these enzymes and changes the carbon flow from the Calvin cycle to oxidative pentose phosphate cycle in the dark conditions.

KEY WORDS: CP12, regulation, cyanobacteria, Calvin cycle