Anacystis nidulans cells grown under high (3%) CO(2) partial pressure have greater phycocyanin to chlorophyll ratio (Phc/Chl) relative to cells grown under low (0.2%) CO(2) tension (Eley (1971) Plant Cell Physiol 12: 311-316). Absorbance difference spectrophotometry of A. nidulans thylakoid membranes in the ultraviolet (DeltaA(320)) and red (DeltaA(700)) regions of the spectrum reveal photosystem II/photosystem I (PSII/PSI) reaction center ratio (RCII/RCI) changes that parallel those of Phc/Chl. For cells growing under 3% CO(2), the Phc/Chl ratio was 0.48 and RCII/RCI = 0.40. At 0.2% CO(2), Phc/Chl = 0.38 and RCII/RCI = 0.24. Excitation of intact cells at 620 nm sensitized RCII at a rate approximately 20 times faster than that of RCI, suggesting that Phc excitation is delivered to RCII only. In the presence of DCMU, excitation at 620 nm induced single exponential RCII photoconversion kinetics, suggesting a one-to-one structural-functional correspondance between phycobilisome and PSII complex in the thylakoid membrane. Therefore, phycobilisomes may serve as microscopic markers for the presence of PSII in the photosynthetic membrane of A. nidulans. Neither the size of individual phycobilisomes nor the Chl light-harvesting antenna of PSI changed under the two different CO(2) tensions during cell growth. Our results are compatible with the hypothesis that, at low CO(2) concentrations, the greater relative amounts of PSI present may facilitate greater rates of ATP synthesis via cyclic electron flow. The additional ATP may be required for the active uptake of CO(2) under such conditions.
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