EXON SHUFFLING AND THE ORIGIN OF THE MITOCHONDRIAL TARGETING FUNCTIONIN PLANT CYTOCHROME C1 PRECURSOR

Since most of the examples of ''exon shuffling'' are between vertebrat e genes, the view is often expressed that exon shuffling is limited to the evolutionarily recent lineage of vertebrates. Although exon shuff ling in plants has been inferred from the analysis of intron phases of plant genes [Long, M., Rosenberg, C. & Gilbert, W. (1995) Proc. Natl. Acad. Sci. USA 92, 12495-12499] and from the comparison of two functi onally unknown sunflower genes [Demon, C. & Steinmetz, A. (1994) Mol. Gen. Genet. 244, 312-317], clear eases of exon shuffling in plant gene s remain to be uncovered. Here, we report an example of exon shuffling in two important nucleus-encoded plant genes: cytosolic glyceraldehyd e-3-phosphate dehydrogenase (cytosolic GAPDH or GapC) and cytochrome c l precursor. The intron-exon structures of the shuffled region indicat e that the shuffling event took place at the DNA sequence level. In th is case, we can establish a donor-recipient relationship for the exon shuffling. Three amino terminal exons of GapC have been donated to cyt ochrome c1, where, in a new protein environment, they serve as a sourc e of the mitochondrial targeting function. This finding throws light u pon an old important but unsolved question in gene evolution: the orig in of presequences or transit peptides that generally exist in nucleus -encoded organelle genes.