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Events Surrounding the Early Development of Euglena Chloroplasts

VII. Photocontrol of the Source Of Reducing Power for Chloramphenicol Reduction by the Ferredoxin-NADP Reductase System ¹

^a Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02154

D(–)threo-Chloramphenicol blocks chlorophyll and plastid protein synthesis in Euglena. During chloroplast development in white light, but not in red, the cells escape from chloramphenicol inhibition and chlorophyll formation is restored. Concomitantly, chloramphenicol is reduced. Reduction of chloramphenicol in an enzyme extract from Euglena requires NADPH and ferredoxin for maximal activity. Methyl viologen replaces ferredoxin, and when chemically reduced, ferredoxin or methyl viologen reduces chloramphenicol directly. This suggests that the enzyme involved is ferredoxin-NADP reductase. In agreement, crude extracts from wild type and W₃BUL, a mutant lacking detectable plastids and plastid DNA, when separated on acrylamide gels, show a single band which reduces methyl viologen with NADPH, and its mobility is similar in wild type and in mutant W₃BUL. The reductase is inducible by light and increases 3-fold in wild type in white or red light and 1.5-fold in W₃BUL in white light. DCMU does not block chloramphenicol reduction in vivo indicating that electrons originate from sources other than photosynthetic electron transport. We infer that chloramphenicol is reduced by ferredoxin which receives electrons via ferredoxin-NADP reductase. The limiting step is not the enzyme but the source of reducing power which can be supplied from the cytoplasm, probably under control of the blue light receptor. Ferredoxin and ferredoxin NADP reductase appear to be coded in the nuclear genome, synthesized on cytoplasmic ribosomes, and join a group of enzymes which cannot be precisely localized, since they may be active anywhere from their site of synthesis in the cytoplasm to their place of deposition in the chloroplast.

³ Abraham and Etta Goodman Professor of Biology. To whom reprint requests should be directed.

¹ Research was supported by Grant GM 14595 from the National Institutes of Health.