EVOLUTIONARY ORIGIN OF CRYPTOMONAD MICROALGAE - 2 NOVEL CHLOROPLAST CYTOSOL-SPECIFIC GAPDH GENES AS POTENTIAL MARKERS OF ANCESTRAL ENDOSYMBIONT AND HOST-CELL COMPONENTS/

Cryptomonads are complex microalgae which share characteristics of chr omophytes (chlorophyll c, extra pair of membranes surrounding the plas tids) and rhodophytes (phycobiliproteins). Unlike chromophytes, howeve r, they contain a small nucleus-like organelle, the nucleomorph, in th e periplastidial space between the inner and outer plastid membrane pa irs. These cellular characteristics led to the suggestion that cryptom onads may have originated via a eukaryote-eukaryote endosymbiosis betw een a phagotrophic host cell and a unicellular red alga, a hypothesis supported by rRNA phylogenies. Here we characterized cDNAs of the nucl ear genes encoding chloroplast and cytosolic glyceraldehyde-3-phosphat e dehydrogenases (GAPDH) from the two cryptomonads Pyrenomonas salina and Guillardia theta. Our results suggest that in cryptomonads the cla ssic Calvin cycle GAPDH enzyme of cyanobacterial origin, GapAB, is abs ent and functionally replaced by a photosynthetic GapC enzyme of prote obacterial descent, GapC1. The derived GapC1 precursor contains a typi cal signal/transit peptide of complex structure and sequence signature s diagnostic for dual cosubstrate specificity with NADP and NAD. In ad dition to this novel GapC1 gene a cytosol-specific GapC2 gene of glyco lytic function has been found in both cryptomonads showing conspicuous sequence similarities to animal GAPDH. The present findings support t he hypothesis that the host cell component of cryptomonads may be deri ved from a phototrophic rather than a organotrophic cell which lost it s primary plastid after receiving a secondary one. Hence, cellular com partments of endosymbiotic origin may have been lost or replaced sever al times in eukaryote cell evolution, while the corresponding endosymb iotic genes (e.g., GapC1) were retained, thereby increasing the chimer ic potential of the nuclear genome.