Experimental reconstruction of the functional transfer of intron-containing plastid genes to the nucleus

Eukaryotic cells arose through the uptake of bacterial endosymbionts and their gradual conversion into cell organelles (mitochondria and chloroplasts). In this process, a massive transfer of genes from the genome of the endosymbiont to the nuclear genome of the host cell occurred. Whereas intron-free organellar genes could conceivably enter the nucleus as DNA pieces and become functional nuclear genes, the transfer mechanisms of organellar genes containing prokaryotic-type group I or group II introns are not clear.

We describe an experimental system that allows us to screen for functional endosymbiotic gene transfer of intron-containing chloroplast genes to the nuclear genome. To distinguish between DNA-mediated and RNA/complementary DNA-mediated transfer, we have constructed an antibiotic resistance gene that is interrupted by a chloroplast group II intron and whose expression is dependent upon both intron removal and gene transfer from the chloroplast genome to the nuclear genome. Screening chloroplast-transformed tobacco plants for the acquisition of the antibiotic resistance via gene transfer to the nucleus, a large number of transfer events were selected. We show that all events involved the direct DNA-mediated transfer of the intron-containing chloroplast gene into the nuclear genome. Gene activity in the nucleus is brought about by utilization of cryptic splice sites within chloroplast intron sequences resulting in appearance of a contiguous reading frame.

Our data pinpoint mechanisms for the functional transfer of organellar genes to the nucleus and demonstrate that intron possession is not an insurmountable obstacle to DNA-mediated endosymbiotic gene transfer.