A NUCLEAR GENE OF EUBACTERIAL ORIGIN IN EUGLENA-GRACILIS REFLECTS CRYPTIC ENDOSYMBIOSES DURING PROTIST EVOLUTION

Genes for glycolytic and Calvin-cycle glyceraldehyde-3-phosphate dehyd rogenase (GAPDH) of higher eukaryotes derive from ancient gene duplica tions which occurred in;eubacterial genomes; both were transferred to the nucleus during the course of endosymbiosis. We have cloned cDNAs e ncoding chloroplast and cytosolic GAPDH from the early-branching photo synthetic protist Euglena gracilis and have determined the structure o f its nuclear gene for cytosolic GAPDH. The gene contains four introns which possess unusual secondary structures, do not obey the GT-AG rul e, and are flanked by 2- to 3-bp direct repeats, A gene phylogeny for these sequences in the context of eubacterial homologues indicates tha t euglenozoa, like higher eukaryotes, have obtained their GAPDH genes from eubacteria via endosymbiotic (organelle-to-nucleus) gene transfer . The data further suggest that the early-branching protists Giardia l amblia and Entamoeba histolytica-which lack mitochondria-and portions of the trypanosome lineage have acquired GAPDH genes from eubacterial donors which did not ultimately give rise to contemporary membrane-bou nd organelles, Evidence that ''cryptic'' (possibly ephemeral) endosymb ioses during evolution may have entailed successful gene transfer is p reserved in protist nuclear gene sequences.