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

Abstract

ABSTRACr Coba_min is esental for growth of Eagkea grclis and photosyn-thesis. Methyloblmin in Eugkla chioroplasts (Y Isegwa, Y Nakano, S Kitaoka, 1984 Plant Physiol 76: 8144818) functions as a coenzyme of methionin synthetase. The requirement of cobalamin for photosynthesis appered remarkably high in Eagkea grown under the dark-precultured condition. The required amount of cobaamin for normal photosynthetic activity was 7A x 10-,' molar, while 7A x 1l0' molar cobalamin was required for normal growth. The lowered photosynthetic activity in cob-alamin-imited cells was restored 20 hos after feeding cyanocbalamin 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 methiine serves for the stbilizaion of pho-tosystem I. This paper is the first report of the physiologial function of o in chkoplasts of photosynthetic eukaryotes. In Euglena gracilis the physiological function of Cbl2 has not been well understood, although Cbl is essential for growth, is rapidly accumulated in the celLs (2, 18) and is converted to