Modulation of chlorophyll synthesis and its effect on the biogenesis of two photosystems

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1. Physiological roles of the two protochlorophyllide reduction systemsCyanobacteria have two protochlorophyllide (Pchlide) reductases catalyzing the conversion of Pchlide to chlorophyllide, a key step in the biosynthetic pathway of chlorophyll (Chl) ; a light-dependent (LPOR) and a light-independent (DPOR) reductase. Gene (por) encoding LPOR has been cloned from Plectonema boryanum, and a por-disrupted mutant, YFP12, was isolated by target mutagenesis to compare phenotype of YFP12 with those of wild-type and a chlL-disrupted (DPOR-less) mutant, YFC2. YFP12 grew as well as wild type under low light conditions, but not under high light conditions (>130 mu E m^<-2>s^<-1>). In contrast, YFC2 was able to grow under both light conditions. These results suggested that, although both the reductases contributed to Chl synthesis in the cells growing in the light, LPOR was essential for the cells to grow under the high light intensity.2. Analysis of the construction of the photosystems under a limitation of Chl availability.YFC2 lacking DPOR was etiolated to reduce the Chl content less than 0.5% of the original level when this mutant was continued to grow under dark for a number of generations. Under this etiolating process, we analyzed the construction of two photosystems, PSI and PSII.Polypeptides of PSII,CP43 and 33 kDa subunit were contained at the level comparable to wild type, while those of PSI,PsaA/PsaB and PsaC were reduced an undetectable level by western analysis. This results suggested that the limitation of Chl supply resulted in the preferential degradation of PSI.