A novel type of chloroplast GAPDH gene in Chlorarachnion
Petersen, J.1, McFadden2, G., Cerff, R.1
1Institut für Genetik, Spielmannstrasse 7, D-38106 Braunschweig, Germany
2School of Botany, Parkville Victoria 3052, Melbourne, Australia
Chlorarachniophytes are amoeboflagellates with complex plastids surrounded by four membranes. The plastids of these microalgae probably evolved via secondary endosymbiosis of a green algae in which chlorophylls a and b are present. Like Cryptomonads they contain a nucleomorph in the periplastidial space between the inner and outer membrane pairs that represents the vestigial nucleus of the engulfed green algae. Glyceraldehyde-3-phosphate dehydrogenases (GAPDH) play key roles in the primary metabolism. They are marker enzymes of two metabolic pathways, the photosynthetic Calvin Cycle and glycolysis. All known eukaryotic crown group organisms contain exclusively GAPDH genes of the GapC and GapAB type.
In Chlorarachnion CCMP621 we found cDNAs of the cytosolic GapC gene and a putative chloroplast GAPDH gene. The latter one is completely different from all known eukaryotic GAPDH genes and is the first member of a novel type GAPDH. Here we present a comparative sequence analysis.
GAPDH genes seems to be good phylogenetic markers to specify ancestral endosymbiont and host cell components. For maintenance of a functional plastid the evolutionary tools of gene transfer or replacement were used individually. In Euglena the plastidial GapAB was successfully transfered to the nucleus, in plastids of Cryptophytes the genes were replaced by a preexisting cytosolic form and in Chlorarachniophytes a novel type GAPDH with unknown origin was recruited. Evolution played dice with the fate of plastidial genes during secondary endosymbiosis.
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