EVIDENCE FOR A CHIMERIC NATURE OF NUCLEAR GENOMES - EUBACTERIAL ORIGIN OF EUKARYOTIC GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE GENES

Higher plants possess two distinct, nuclear gene-encoded glyceraldehyd e-3-phosphate dehydrogenase (GAPDH) proteins, a Calvin-cycle enzyme ac tive within chloroplasts and a glycolytic enzyme active within the cyt osol. The gene for the chloroplast enzyme was previously suggested to be of endosymbiotic origin. Since the ancestors of plastids were relat ed to cyanobacteria, we have studied GAPDH genes in the cyanobacterium Anabaena variabilis. Our results confirm that the nuclear gene for hi gher plant chloroplast GAPDH indeed derives from the genome of a cyano bacterium-like endosymbiont. But two additional GAPDH genes were found in the Anabaena genome and, surprisingly, one of these sequences is v ery similar to nuclear genes encoding the GAPDH enzyme of glycolysis i n plants, animals, and fungi. Evidence that the eukaryotic nuclear gen es for glycolytic GAPDH, as well as the Calvin-cycle genes, are of eub acterial origin suggests that eukaryotic genomes are more highly chime ric than previously assumed.