Résumé
The photosynthetic protist
Euglena gracilis contains chloroplasts surrounded by three membranes which arise from secondary endosymbiosis. The genes
petA and
petD, encoding cytochrome
f and subunit IV of the cytochrome
bf complex, normally present in chloroplast genomes, are lacking from the chloroplast DNA (cpDNA) of
E. gracilis. The
bf complex of
E. gracilis was isolated, and the identities of cytochrome
f and subunit IV were established immunochemically, by heme-specific staining, and by Edman degradation. Based on N-terminal and conserved internal protein sequences, primers were designed and used for PCR gene amplification and cDNA sequencing. The complete sequence of the
petA cDNA and the partial sequence of the
petD cDNA from
E. gracilis are described. Evidence is provided that in this protist, the
petA and
petD genes have migrated from the chloroplast to the nucleus. Both genes exhibit a typical nuclear codon usage, clearly distinct from the usage of chloroplast genes. The
petA gene encodes an atypical cytochrome
f, with a unique insertion of 62 residues not present in other
f-type cytochromes. The
petA gene also acquired a region that encodes a large tripartite chloroplast transit peptide (CTP), which is thought to allow the import of apocytochrome
f through the three-membrane envelope of
E. gracilis chloroplasts. This is the first description of
petA and
petD genes that are nucleus-localized.