Quaternary structure of the d ‐ribulose 1,5‐diphosphate carboxylase from the cyanelles of Cyanophora paradoxa

It is well established that i)-ribulose 1,5-diphosphate (RuDP) carboxylase from the chloroplasts of phototrophic eukaryotes is of high molecular weight (about 550 000) and consists of two classes of subunits of approximately 55 000 (large, L) and 15 000 (small, S), arranged in an 8 L + 8 S quaternary form [ 1 4]. Greater structural variation has been found between RuDP carboxylases from autotrophic prokaryotes. The enzymes from the photosynthetic bacteria Chrornatium [5] and Ectothiorhodospira halophila [6] and the hydrogen bacterium Alcaligenes eutrophus [7,8] closely resemble the eukaryotic type in molecular weight and subunit composition (8 L + 8 S). However, low molecular weight forms of bacterial RuDP carboxylase, containing only L subunits, have also been found, namely of 114 000 (Rhodospirillum nlbrum, ref. [9]) and of about 350 000 (Ozlorobium limicola f thiosulfatophilum, ref. [10]). The RuDP carboxylases of blue-green algae are of interest in view of the possible endosymbiotic origin of chloroplasts from an ancestral oxygen-evolving autotrophic prokaryote [1113]. The enzymes from Anabaena variabilis, l'lectonema boryanum and Aphanocapsa 6308 are similar to the typical chloroplast RuDP carboxylase in molecular weight and quaternary structure, possessing L and S subunits [14,15]. An endosymbiotic origin has been suggested for the cyanelles of the biflagellate protist Cyanophora paradoxa [16,17], since the cyanelles are similar to unicellular blue-green algae in structure [17] and pigment composition [18]. The present communication reports on the purification and partial characterization of the cyanelle RuDP carboxylase, which is cornpared with the enzymes from free-living blue-green algae and chloroplasts.