Energy Supply for ATP-Synthase Deficient Chloroplasts of Chlamydomonas reinhardii

Abstract

The mutant F54 of the unicellular green alga Chlamydomonas reinhardii is not able to perform photophosphorylation. Nevertheless, it grows on acetate and the chloroplasts accomplish most of their energy-requiring synthetic processes. However, no light-dependent chloroplast protein synthesis could be detected in intact F54 chloroplasts isolated from a cell wall-deficient double mutant F54-cw-15. Exogenous ATP was not able to induce this in organello protein synthesis to an appreciable degree. In contrast, the strictly ATP-dependent protein synthesis was stimulated very efficiently by glyceraldehyde-3-phosphate, dihydroxyacetone phosphate and glycerol-3-phosphate, but strongly inhibited by 3-phosphoglycerate. These compounds can be transported across the envelope membrane by the triose phosphate translocator. Pyridoxal phosphate, a specific inhibitor of the translocator, abolished the stimulation by triose phosphates. Spermidine, which activates initiation of translation in chloroplasts, enhanced triose phosphate-stimulated protein synthesis even further. In the dark, no stimulation was observed, indicating that a light-dependent reaction was also involved in this kind of ATP production in chloroplasts. The results suggest that chloroplasts defective in photophosphorylation recruit their energy via an ATP shuttle which was shown in this study to import rather than export ATP across the chloroplast envelope.