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Cobalamin is essential for growth of Euglena gracilis and photosynthesis. Methylcobalamin in Euglena chloroplasts (Y Isegawa, Y Nakano, S Kitaoka, 1984 Plant Physiol 76: 814-818) functions as a coenzyme of methionine synthetase. The requirement of cobalamin for photosynthesis appeared remarkably high in Euglena grown under the dark-precultured condition. The required amount of cobalamin for normal photosynthetic activity was 7.4 x 10(-11) molar, while 7.4 x 10(-10) molar cobalamin was required for normal growth. The lowered photosynthetic activity in cobalamin-limited cells was restored 20 hours after feeding cyanocobalamin or methionine to cobalamin-limited cells. Lowering of photosynthetic activity was due to loss of photosystem I activity. This photosynthetic activity was recovered after supplementation by methionine or cobalamin. The results suggest that methionine serves for the stabilization of photosystem I. This paper is the first report of the physiological function of cobalamin in chloroplasts of photosynthetic eukaryotes.

Photosynthesis of Euglena gracilis under Cobalamin-Sufficient and -Limited Growing Conditions