TY - JOUR
T1 - The Cyanelles of Cyanophora paradoxa
AU - Löffelhardt, W.
AU - Bohnert, H. J.
AU - Bryant, D. A.
PY - 1997
Y1 - 1997
N2 - The cyanelles of Cyanophora paradoxa, plastids surrounded by a peptidoglycan wall, are considered as a surviving example for an early stage of plastid evolution from endosymbiotic cyanobacteria. We highlight the model character of the system by focusing on three aspects: "organelle wall" structure, plastid genome organization, and protein translocation. The biosynthetic pathway for cyanelle peptidoglycan appears to be analogous to that in Escherichia coli. Also, the basic structure of this peculiar organelle wall corresponds to that of the E. coli sacculus, with one notable exception: the C-1 carboxyl group of the D-isoglutamyl residue is partially amidated with N-acetylputrescine. Cyanelles harbor on their completely sequenced 135.6-kb genome genes for approximately 150 polypeptides, many of which are nucleus encoded in higher plants. Nevertheless, there are striking parallels in genome organization between cyanelles (and other primitive plastids) and higher plant chloroplasts. The transit sequences of nucleus-encoded cyanelle preproteins resemble stroma targeting peptides of higher plant chloroplast precursors. Heterologous import of precursors from C. paradoxa into isolated pea chloroplasts is possible and vice versa. Cyanelles are considered to represent a very early, diverging branch of plastid evolution and are derived from the semiautonomous endosymbiont that had already abandoned about 90% of its genetic information but still retained its prokaryotic wall. Recent data on the molecular biology of cyanelles and rhodoplasts are consistent with the assumption of a primary endosymbiotic event that was not only monophyletic with respect to the cyanobacterial 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 morphology and organization of the "host cells" (Kies, 1992). Recently, this was corroborated by 16S and 18S rRNA-based phylogenetic analysis (Helmchen et al., 1995; Bhattacharya et al., 1995). Apart from C. paradoxa, 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) that is kept in the major culture collections of algae. Biochemical work done on C. paradoxa and the sequencing of individual cyanelle genes have been described in several recent reviews (Schenk, 1992; Löffelhardt and Bohnert, 1994a,b). Here we discuss three topics: the cyanelle wall, aspects deduced from the complete cyanelle genome sequence, and protein translocation into and within cyanelles.
AB - The cyanelles of Cyanophora paradoxa, plastids surrounded by a peptidoglycan wall, are considered as a surviving example for an early stage of plastid evolution from endosymbiotic cyanobacteria. We highlight the model character of the system by focusing on three aspects: "organelle wall" structure, plastid genome organization, and protein translocation. The biosynthetic pathway for cyanelle peptidoglycan appears to be analogous to that in Escherichia coli. Also, the basic structure of this peculiar organelle wall corresponds to that of the E. coli sacculus, with one notable exception: the C-1 carboxyl group of the D-isoglutamyl residue is partially amidated with N-acetylputrescine. Cyanelles harbor on their completely sequenced 135.6-kb genome genes for approximately 150 polypeptides, many of which are nucleus encoded in higher plants. Nevertheless, there are striking parallels in genome organization between cyanelles (and other primitive plastids) and higher plant chloroplasts. The transit sequences of nucleus-encoded cyanelle preproteins resemble stroma targeting peptides of higher plant chloroplast precursors. Heterologous import of precursors from C. paradoxa into isolated pea chloroplasts is possible and vice versa. Cyanelles are considered to represent a very early, diverging branch of plastid evolution and are derived from the semiautonomous endosymbiont that had already abandoned about 90% of its genetic information but still retained its prokaryotic wall. Recent data on the molecular biology of cyanelles and rhodoplasts are consistent with the assumption of a primary endosymbiotic event that was not only monophyletic with respect to the cyanobacterial 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 morphology and organization of the "host cells" (Kies, 1992). Recently, this was corroborated by 16S and 18S rRNA-based phylogenetic analysis (Helmchen et al., 1995; Bhattacharya et al., 1995). Apart from C. paradoxa, 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) that is kept in the major culture collections of algae. Biochemical work done on C. paradoxa and the sequencing of individual cyanelle genes have been described in several recent reviews (Schenk, 1992; Löffelhardt and Bohnert, 1994a,b). Here we discuss three topics: the cyanelle wall, aspects deduced from the complete cyanelle genome sequence, and protein translocation into and within cyanelles.
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U2 - 10.1080/07352689709701955
DO - 10.1080/07352689709701955
M3 - Article
AN - SCOPUS:0000275443
SN - 0735-2689
VL - 16
SP - 393
EP - 413
JO - Critical Reviews in Plant Sciences
JF - Critical Reviews in Plant Sciences
IS - 4
ER -