Aspects of the relation between Cyanophora paradoxa (Korschikoff) and its endosymbiotic cyanelles Cyanocyta korschikoffiana (Hall & Claus) - I. Growth, ultrastructure, photosynthesis and the obligate nature of the association

Ultrastructural studies of the symbiotic relation between the apochlorotic cryptomonad Cyanophora paradoxa and the cyanobacterium Cyanocyta korschikoffiana have demonstrated that the endosymbiotic cyano­bacteria are within host cell vacuoles and possess a highly reduced prokaryotic cell wall which becomes particularly evident during binary fission. Division by the endosymbiotic cyanobacteria was found not to be in synchrony with division by the host cryptomonad. The cyano­bacteria divided most rapidly when the host population had reached its asymptote, such that, in cultures in extended stationary phase, it is possible to observe as many as eight cyanobacteria in one host cell. Nutritionally, C. paradoxa was found to be an obligate phototroph; its auxotropic capabilities are confined to acetate and cyanocobalamin. Analysis of the rate of photosynthesis by the intact association demon­strated that saturation occurred at the 140 μE cm -2 s -1 (where E is einstein), and at saturating light intensities, the rate of CO 2 fixation was 30 μmol CO 2 (mg chl a ) -1 h -1 . The best rate of CO 2 fixation obtained by the isolated cyanobacteria represented 12% of the rate found in the intact association. Two independent methods of analysis showed that at best, only 15% of the carbon fixed by the cyanobacteria was translocated to the host, mostly as glucose and probably sucrose. Studies with metabolic inhibitors directed against the prokaryotic cyanobacteria demonstrated the close interdependence of the host with the endosymbionts, in that disruption of the endosymbionts always resulted in death of the host. Our inability to grow the cyanobacteria away from the host, combined with the apparent inability of the host to survive in the absence of the photosynthetic endosymbiotic cyanobacteria provide strong support for the concept that the association represents a well integrated mutually obligate relation, but the concept that the cyanobacteria are chloroplasts cannot be supported.

The biogenesis of the cyanellae of Cyanophora paradoxa. II. Pulse-labelling of cyanellar polypeptides in the presence of transcriptional and translational inhibitors

Cycloheximide and chloroamphenicol, specific inhibitors of protein translation in the cytoplasmic and cyanellar compartments, respectively, of Cyanophora paradoxa, have been employed in 30 min pulse-labelling experiments by using [NaH-14C]O3 to label total cell proteins in vivo. Cyanellae purified from host cell lysates were separated into soluble and thylakoid fractions and analysed by one- and two-dimensional polyacrylamide gel electrophoresis (PAGE) to determine the distribution of radioactivity in the cyanellar polypeptides. Analysis of the autoradiograms of electrophoretically resolved proteins of the cyanellae indicates that about 70% of the total number of cyanellar proteins visualized in the controls are synthesized on cytoplasmic ribosomes. The majority (81%) of the soluble cyanellar proteins appear to be cytoplasmically synthesized. In contrast, the majority (70%) of the thylakoid proteins are synthesized within the cyanellae. The observations also suggest that the polypeptides synthesized within the cyanellae include species that are the most abundant and rapidly turned over. A number of the polypeptides previously identified have now been characterized with regard to their sites of synthesis. In addition, we report on labelling experiments involving rifampicin, a specific inhibitor of cyanellar transcription, which indicate that different mRNAs within the cyanellae have markedly different stabilities.

Metabolic activities in Cyanophora paradoxa and its cyanelles : II. Photosynthesis and respiration

Isolated cyanelles of Cyanophora paradoxa perform photosystem I and II dependent Hill reactions. The photosynthetic electron transport of the cyanelles does not show special features uncommon in cyanobacteria or chloroplasts of red algae. A preparation of cyanelles performs photosynthetic O2-evolution with approximately 1/3 of the rate of intact Cyanophora, in only, however, the first three minutes of the experiment. All attempts to stabilize the CO2-fixation activity of isolated cyanelles failed. Isolated cyanelles do not perform KCN-sensitive O2-uptake, indicating that respiratory cytochrome oxidase is lacking in cyanelles. O2-consumption by crude extracts from Cyanophora is inhibited by KCN when N-tetramethyl-p-phenylenediamine/ascorbate or NADH but not NADPH are supplied as the electron donors in contrast to the situation in cyanobacteria. These findings suggest that cyanelles do not respire. It is concluded that cyanelles are not so much related to cyanobacteria as formerly believed, but share many properties with chloroplasts of eukaryotic cells.

On the developmental dependence between Cyanophora paradoxa and Cyanocyta korschikoffiana in symbiosis : Host-dependent development of endocyanelles

The prokaryote Cyanocyta korschikoffiana was isolated from the eukaryote Cyanophora paradoxa. The synthesis of several thylakoid proteins in these cyanelles is influenced by light and darkness and is sensitive to cycloheximide, the inhibitor of the eukaryotic host's translation. The possibility of a direct coordination between the translations of the host and of the cyanelles is discussed.