Functional Analysis of Isozymes of Light-dependent Protochlorophyllide Oxidoreductase on Chloroplast Biogenesis

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The reduction of protochlorophyllide (Pchlide) is a key regulatory step in the biosynthesis of chlorophyll in phototrophic organisms. In angiosperms, POR gene family is differentially represented, and gene expression of POR isoform is differentially regulated by light and development.To elucidate the physiological function of three differentially regulated POR isoforms (PORA, PORB and PORC) in Arabidopsis thaliana, we analyzed null mutants of porB and porC. When illuminated, etiolated porB seedling was able to green to the similar extent to the wild type under high and medium light conditions, while the greening was significantly reduced under low light condition. During greening, the high light irradiation increased the level of PORC protein, and the greening of porC was repressed under the high light condition. These results suggest that as well as PORA, PORB is also function for PLBs assembly and photoactive Pchlide formation. It is likely that, at onset of greening, PLBs are import … More ant for efficient capturing of light energy for photoconversion under various light conditions, and PORC, which is induced by high light irradiation, contributes to photoprotection.Meanwhile, cytokinins and light activate the transcription of the cucumber POR gene. To identify the cytokinin-responsive cis-element corresponding to chlorophyll biosynthesis and chloroplast development, we performed transient expression assays in etiolated cucumber cotyledons. A5'-deletional analysis indicated that the promoter region contained at least one of the cis-elements related to cytokinin-responsiveness. Gel mobility shift assays also detected cytokinin-enhanced binding in this region. More detailed gel mobility shift and competition analyzes identified 5'-TATTAG-3' as the sequence critical for cytokinin-enhanced binding. Mutations in the identified sequence in the transient expression assay caused a reduced but retained cytokinin-responsiveness, as well as low reporter activity of untreated control. These results suggest that the identified sequence is a novel cis-element exhibiting cytokinin-dependent protein binding in vitro, which may function effectively when interacting with other cytokinin-related elements. Less