THE CYANELLES OF CYANOPHORA-PARADOXA

The cyanelles of Cyanophora paradoxa, plastids surrounded by a peptido glycan wall, are considered as a surviving example for an early stage of plastid evolution from endosymbiotic cyanobacteria. We highlight th e model character of the system by focusing on three aspects: ''organe lle wall'' structure, plastid genome organization, and protein translo cation. The biosynthetic pathway for cyanelle peptidoglycan appears to be analogous to that in Escherichia coli. Also, the basic structure o f this peculiar organelle wall corresponds to that of the E. coli sacc ulus, with one notable exception: the C-l carboxyl group of the D-isog lutamyl residue is partially amidated with N-acetylputrescine. Cyanell es harbor on their completely sequenced 135.6-kb genome genes for appr oximately 150 polypeptides, many of which are nucleus encoded in highe r plants. Nevertheless, there are striking parallels in genome organiz ation between cyanelles (and other primitive plastids) and higher plan t chloroplasts. The transit sequences of nucleus-encoded cyanelle prep roteins resemble stroma targeting peptides of higher plant chloroplast precursors. Heterologous import of precursors from C. paradoxa into i solated pea chloroplasts is possible and vice versa. Cyanelles are con sidered to represent a very early, diverging branch of plastid evoluti on and are derived from the semiautonomous endosymbiont that had alrea dy abandoned about 90% of its genetic information but still retained i ts prokaryotic wall. Recent data on the molecular biology of cyanelles and rhodoplasts are consistent with the assumption of a primary endos ymbiotic event that was not only monophyletic with respect to the cyan obacterial invader, but also singular. Cyanophora paradoxa is the best -investigated member of the glaucocystophyceae, phototrophic protists containing cyanelles, that is, plastids stabilized by a peptidoglycan- containing envelope. The classification of this group, comprising only eight (mostly monotypic) genera, is also based on parallels in morpho logy and organization of the ''host cells'' (Kies, 1992). Recently, th is was corroborated by 16S and 18S rRNA-based phylogenetic analysis (H elmchen et at, 1995; Bhattacharya et al., 1995). Apart from C. paradox a, only Glaucocystis nostochinearum can be grown at a reasonable rate. Thus, biochemical and molecular genetic data are mostly available for C. paradoxa and more precisely for the isolate 555UTEX (Pringsheim) t hat is kept in the major culture collections of algae. Biochemical wor k done on C. paradoxa and the sequencing of individual cyanelle genes have been described in several recent reviews (Schenk, 1992; Loffelhar dt and Bohnert, 1994a,b). Here we discuss three topics: the cyanelle w all, aspects deduced from the complete cyanelle genome sequence, and p rotein translocation into and within cyanelles.