EVOLUTION AND BIOLOGY OF CYANELLES J. Steiner, B. Pfanzagl, Y. Ma and W. L6ffelhardt J.Steiner,B.Pfanzagl, Y.Ma and W. Lbffelhardt (corresponding author; e-mail: wlabc.univie. ac.at),Institut fur Biochemie und Molekulare Zellbiologie der Universitat Wien andLudwig-Boltzmann Forschungsstelle fOr Biochemie, Dr. Bohrgasse 9,A-1030 Vienna, Austria. INTRODUCTION Cyanelles are the primitive plastids of glaucocysto phyte algae and are distinguished by the possession of a peptidoglycan wall. This prokaryotic feature, not found in any other plastid types, clearly indi cates plastid evolution from an ancestral cyanobac terial endosymbiont. Comparative analyses of the genomes of cyanelles, rhodoplasts and choroplasts, as well as investigations of the respective protein import apparatus, have led to the assumption of a singular primary endosymbiotic event. The term cyanelle was originally used to des ignate endosymbiotic cyanobacteria (P1. I) con tained in a variety of host cells, e.g. biflagellated protists (Cyanophora paradoxa), thekamoebae (Paulinella chromatophora) and fungi (Geosiphon pyriforme). Taxonomic assignments were changed several times, especially for the best-investigated 'endocyanome', C. paradoxa. It was only in the past decade, with the application of molecular methods, that a clear classification was made. First, it became obvious that the cyanelle from C. para doxa is in fact an organelle -a primitive plastid and not an endosymbiotic cyanobacterium. In contrast, the filamentous cyanobacterial symbionts in the bladders formed by G. pynforme were recognised as Nostoc sp., which could be cultivated indepen dently. Most of the remaining cyanelle-containing organisms have been combined to form the Glaucocystophyceae, as proposed by L. Kies (Table 1). This small group comprises eight genera, most of them monotypic. The unifying characteristics are the presence of peptidoglycan-surrounded cyanelles (Plate 1), the structural similarity of the flagellar roots and the presence under the plasma membrane of microtubule-associated vesicles (lacu nae) that in several species contain scale-like cellu losic plates (Kies 1992). This classification was corroborated by phylogenetic analysis based on 16S and 18S rRNA sequences from C. paradoxa, Glaucocystis nostochinearum and Gloeochaete wittrocki ana (Bhattacharya and Schmidt 1997). The thekamoeba P. chromatophorawas excluded from the Glaucocystophyceae because of differences in morphology and molecular data; however, it too seems to contain cyanelles, probably resulting from a secondary endosymbiotic event inwhich a glauco cystophycean alga was taken up (D. Bhattacharya, pers. comm.). Cyanelle division is sensitive to P-lactam antibiotics, owing to the presence of seven peni cillin-binding proteins in the size range 30 llOkDa (Ldffelhardt et al. 1997). A thorough structural characterisation of cyanelle peptidoglycan revealed extensive amidation of the C-1 carboxy group in the D-isoglutamyl moiety of the peptide side chains by N-acetylputrescine. This substitu tion was not found in the peptidoglycan from the cyanobacterium Synechocystis sp. PCC 6714 but was found in the cyanelle sacculi from G. nostochinearum and Cyanoptyche gloeocystis. The net charge of each peptide side chain is thereby re duced from - 2 to - 1 (Lbffelhardt et al. 1997). THE RELATIONSHIP BETWEEN CYANELLES AND PLASTIDS Comparison of the organisation of the 136kb cyanelle genome with that of other completely secwenced plastid genomes and with Synechocystis sp. PCC 6803 yielded the following information (Ldffelhardt et al. 1997): (i) The cyanelle genome contains approximately 40 additional protein genes not present in the chloroplast genomes from higher plants, i.e. a primitive plastid contains a higher number of genes than a denrved plastid. (ii) Transcription units known from cyanobacteria are often well preserved in chloroplast (and especially the cyanelle) genomes, i.e. plastid genome organisation shows distinct prokary otic features. (iii) On the other hand, some gene clusters that exist in cyanelles, rhodoplasts and chloroplasts have no parallel among cyanobacteria, e.g. 5'-rpoB-rpoCl-rpoC2-rps2-atpH-atpG-atpF-atpD atpA -3' In the latter case, the genes are specifying subunits of three different enzyme or macromolecular com plexes-the ribosome, ATP synthetase and RNA polymerase. Therefore, no selective pressure to form or maintain such a transcription unit is pos sible. Indeed, in the Chlamydomonas reinhardtii chloroplast, notorious for its ability for rearrange ments, this cluster appears to be disrupted. Conse quently, when we exclude convergent evolution, BIOLOOY AND ENVIRONMENT: PROCEEDINGS OF THE ROYAL IRISHACADEMY, VOL. 102B, No. 1, 7-9 (2002...