Characterization and Evolution of a Chloroplastic Gene That Was Functionally Transferred to the Nucleus

Eukaryotic cells arose more than a billion years ago through endosymbiotic engulfment of free-living bacteria that were gradually converted into mitochondria and chloroplasts respectively. Since these events, there has been a continuous deluge of organellar DNA entering the nuclear genome. This is a source of genetic diversity and has created genes or nuclear exons encoding parts of novel proteins. Chloroplastic genes (prokaryotic) must acquire some nuclear regulatory elements, such as a promoter or a polyadenilation signal, in order to become functional in the nuclear environment. The recent sequencing of T. caeruleum (Blue throatwort, Asterales) plastome showed that the essential accD gene (acetyl-coA carboxylase) has been lost and thus must have been been relocated to the nucleus. We found experimentally a nuclear transcript related to chloroplastic accD in T. caeruleum. It contains the carboxylase domain of the chloroplast accD gene and nuclear sequences containing regulatory elements. The predicted presence of a chloroplast-directed transit peptide-encoding sequence at the 5’end of the nuclear accD transcript was verified experimentally. We also introduced a construct containing the nuclear accD transcript into a mutant tobacco plant heteroplastomic for a deletion of the chloroplastic accD gene to determine whether this nuclear version can replace the chloroplast gene. The origin of the loss of the chloroplastic accD was also determined by performing a slot-blot experiment using total cellular DNA of more than 20 species of the Asterales. The evolution of the nuclear gene (coding and intronic sequences) since it has been activated has also been studied.