IMMUNOCYTOCHEMICAL LOCALIZATION AND FUNCTIONAL DETERMINATION OF PHYTOENE DESATURASE IN PHOTOAUTOTROPHIC PROKARYOTES

The in situ location of phytoene desaturase, an enzyme in the carotenoid biosynthetic pathway, has been investigated in the photosynthetic organisms Rhodobacter capsulatus SB 1003, Synechocystis PCC 6714, and Anabaena variabilis ATCC 29413. A post-embedding immunogold electron microscopy procedure using a polyclonal antibody against a phytoene desaturase fusion protein was employed. For all three species most of the gold particles (about 85% in the case of Anabaena and Synechocystis) were associated with the photosynthetic membranes. Other parts of the cells were only randomly and not significantly labelled. When the anti-phytoene desaturase antibody was omitted, or when pre-immune serum was used, no gold deposition was observed. In vitro phytoene desaturase reactions, especially with purified thylakoid membranes from Synechocystis, confirmed the nature of phytoene desaturase as an integral protein of the thylakoid membrane.