In a reconstruction of events that took place early in the evolution of eukaryotic cells, a new study has shown how the transfer of DNA from an organelle to the nucleus can give rise to functional genes.

After the initial uptake of endosymbiotic bacteria that led to eukaryotic evolution, DNA transfer took place between newly formed organelles and host nuclear genomes. Stegemann and Bock used cultured tobacco cells carrying a construct that represents the transfer of a plastid gene to a position next to a nuclear gene. The construct was moved from transgenic chloroplasts to the nuclear genome and consists of one gene, aadA , in a cassette with sequences that drive its expression only in plastids, plus an upstream gene, nptII, which is flanked by eukaryotic promoter and termination signals.

By growing large numbers of cells in the presence of the two antibiotics to which aadA encodes resistance, the authors selected lines in which aadA had become functional, with expression from the nuclear genome. In all cases, rearrangements had placed aadA under the control of the nptII promoter; however, the signals that were needed for mRNA 3′ end formation had been provided by the plastid 3′ flanking sequences. The authors postulate that the AT-richness of plastid non-coding regions might make it easier for chloroplast genes to take on eukaryotic-type mRNA cleavage and polyadenylation.

Although other outcomes and functionalization processes are possible, this study provides an important insight into how organellar gene functions can be taken over by the nucleus — a process that might have fundamentally shaped the biology of eukaryotic cells.