Quantitative and compositional changes in monogalactosyl and digalactosyl diglycerides during light-induced formation of chloroplasts in Euglena gracilis.

The formation of chloroplasts in dark-grown cells of Euglena gracilis was induced by exposing the cells to constant illumination. Following a lag, the cells accumulated chlorophyll and galactosyl diglycerides simultaneously at almost linear rates. The monogalactosyl diglyceride content rose from approximately 2 micromoles in 100 mg of dark-grown cells to 27 micromoles in fully green cells; the digalactosyl diglyceride content increased from 1 micromole to 11 micromoles. The digalacto compounds increased more rapidly than the monogalacto compounds at first, but their rate of accumulation began to diminish long before greening of the cell was complete. The sole exception was the digalactosyl diglyceride fraction that contained hexadecadienoic (16:2) fatty acid. This fraction increased continuously during greening. As accumulation of the digalacto compounds diminished, that of the monogalacto compounds increased. Towards the end of greening, the major fatty acids were 16:2, 16:3, 16:4, 18:2, and 18:3 in the monogalacto and 16:2 in the digalacto compounds. The results of this study suggest that monogalactosyl and digalactosyl diglycerides that contain particular fatty acid components have a function in the assembly of chloroplasts.