Is the ´central body´ of the cyanelles from Cyanophora paradoxa an eukaryotic carboxysome?

Is the central body of the cyanelles of Cyanophora paradoxa an eukaryotic carboxysome? Photosynthesis certainly constitutes the basis for life on our planet in its present form. In addition to land plants, cyanobacteria and algae play a major role in global carbon dioxide fixation. Bearing in mind the affinity of the key enzyme Rubisco for its substrate, the CO 2 concentration in the atmosphere (0.035%) in fact has to be considered as "too low". In the course of evolution first cyanobacteria, then algae as their descendants and finally a number of higher plant families developed CO 2 concentration mechanisms (CCMs) to counteract this shortcoming. In phototrophic microorganisms CCMs comprise inducible transport systems for CO 2 and HCO3 - (leading to an accumulation of Ci) und a Rubisco reaction under optimised conditions in a microcompartment: in carboxysomes with cyanobacteria, in pyrenoids with algae. Cyanophora paradoxa is a glaucocystophyte alga containing peculiar plastids: these cyanelles (muroplasts) are confined by a peptidoglycan wall and are thought to be the closest relatives (among plastids) to cyanobacteria. In the center of the cyanelle an electron dense body is located that is surrounded by cyanelle DNA and the concentric thylakoid membranes. Using this system we want to investigate a possible transition stage in the evolution of CCM, i.e. if primitive plastids first took over the carboxysome mechanism of the cyanobacterial endosymbiont and the modification to the pyrenoid-type mechanism occurred later. A number of preliminary findings point to the carboxysome nature of the cyanelle central bodies. In some glaucocystophyte species they are polyhedral and confined by a (proteinaceous?) shell, in C. paradoxa cyanelle DNA is bound to their surface. During cyanelle division the central body is cleaved by the septum. Pyrenoids possess no confining membrane-like structures, are in general penetrated by thylakoid membranes and dissappear during chloroplast division. In any case the accumulated form of Ci is HCO3 - (where the losses through diffusion are neglibible compared to CO 2 ) which has to be converted by carbonic anhydrase (CA) into the Rubisco substrate CO 2 . The intraorganellar localisation of CA defines the microcompartment type. In carboxysomes, CA is co-packaged together with Rubisco. In pyrenoids, CA is found in the lumen of the penetrating thylakoid membranes. The Cyanophora project is planned as a cooperation with the groups of Prof. Hans Bohnert (University of Illinois, Urbana-Champaign) und Prof. Aaron Kaplan (Hebrew University, Jerusalem). Distribution of work will be done according to the expertise and equipment present. The action of a CCM will be confirmed experimentally, central bodies will be isolated and their additional components (other than Rubisco) investigated, as CA and shell proteins. It is also planned to use immuno-electron microscopy. It will be attempted, via PCR, to directly characterise conserved genes für CA, transporter components, shell proteins etc. In parallel, cDNA microarray technology will be exploited, making use of the fact that the expression of almost all genes involved in CCM increases upon lowering the concentration of CO 2 . All genes, the transcripts of which are at higher levels during growth in 0.035% CO 2 than in 2% CO 2 will be surveyed. Thus a certain amount of DNA sequencing will be necessary. We expect to find several interesting genes among the EST`s generated. We want to demonstrate that Cyanophora as a "living fossil" had not only retained the organelle wall und typical cyanobacterial features of cyanelle genome organisation, but had also taken over the cyanobacterial type of CCM into an eukaryotic cell.