Immunocytochemical Localization of Phosphoribulose

The distribution of phosphoribulose kinase (PRK) in the cyanelies of Cyawphora paradoxa Korschikoff and Glucocystis xostochinearwm Itzigsohn was studied by protein A-gold immunoelectron microscopy. In both endocyanomes, antiserum against PRK heavily labeled the thylakoid region of the cyanelles, whereas little or no label was present over the carboxysomes. Antiserum aginst ribulose 1,5-bisphosphate carboxyase/ oxygenase by contrast heavily labeled the carboxysomes of each endocyanome. In vitro studies of PRK distribution in cell-free extacts of C. pardoxa showed that 93% of the enzyme was in the soluble fractin Quantitative immunoelectron mcroscopy showed that more than 99% of the PRK in the cyanelle of C. parwdoxa was lalized in the thylakoid region. We conclude that the carboxysomes of cyanelles like the carboxysomes of autotrophic prokaryotes and the pyrenoids of green algl chloroplasts do not contain phosphoribulose kinase. on the basis of in vitro fractionation studies with both organisms (4, 10, 18) and immunoelectron microscopy of C. fritschii cell sections (7). Electron-dense inclusion bodies are a feature of several cyanelles (6, 1 1). The likely origins ofcyanelles from endosymbiotic ancestral photosynthetic prokaryotes contributes to the interest in these structures, which may represent an intermediate stage in the origins of chloroplasts (8). We have recently shown by immunoelectron microscopy that the central dense body in the cyanelles of Cyanophora paradoxa and the terminal electrondense lamella-free region in the cyanelles of Glaucocystis nostochinearum also contain RuBisCO (16) and thus are probably functionally equivalent to the carboxysomes of autotrophic prokaryotes. In the present study we wished to determine whether or not PRK is present in the carboxysomes of cyanelles. We show by in vitro fractionation of whole cell extracts and by immunoelectron microscopy that PRK is localized throughout the thylakoid region of the cyanelles of C. paradoxa and G. nostochinearum and that no enzyme is localized in the carboxysomes.