Immunogold Localization of Photosynthetic Proteins in Euglena

Abstract

Summary: Changes in pyrenoid morphology and the distribution within the chloroplast of ribulose-1, 5-bisphosphate carboxylase/oxygenase(RuBisCO)of Euglena gracilis Z were followed by immunoelectron microscopy during growth and division phase. Most of the immunoreactive protein was found in the pyrenoid during the growth phase with only a small amount of gold particles localized to the stroma. During the division phase, the pyrenoid was not detected and the gold particles were dispersed throughout the stroma. A comparison between rates of photosynthetic CO2-fixation and the amount of total carboxylase activity catalyzed by RuBisCO extracted from Euglena cells in the growth phase suggests that pyrenoid localized RuBisCO is responsible for photosynthetic CO2-fixation. The precursors to the Euglena light-harvesting chlorophyll a/b binding proteins of photosystem II(LHCP II)are large polyproteins containing multiple copies of LHCP II covalently joined by a conserved decapeptide. Light induced LHCP II synthesis is controlled at the translational level in Euglena rather than the transcriptional level as found in higher plants and other algae. Under conditions promoting LHCP II synthesis and accumulation in the thylakoids, a reaction with anti-LHCP II antibody can be observed by immunogold electron microscopy in the Golgi apparatus. The kinetics of LHCP II appearance in the Golgi apparatus as measured by immunogold electron microscopy in synchronous cells and by pulse labeled with H2[35S]O4 in cells undergoing normal light-induced chloroplast development suggests that nascent LHCP II is transported to the Golgi apparatus prior to chloroplast import and thylakoid insertion.