Fate of mitochondrially located S19 ribosomal protein genes after transfer of a functional copy to the nucleus in cereals.

Mitochondrial genes for ribosomal proteins undergo relatively frequent transfer to the nucleus during plant evolution, and when migration is successful the mitochondrial copy becomes redundant and can be lost. We have examined the status of the mitochondrial rps19 gene for ribosomal protein S19 in closely related cereals. In oat, the mitochondrial rps19 reading frame is blocked by a premature termination codon and lacks abundant transcripts, whereas in the mitochondria of wheat and rye rps19 is a 5'-truncated pseudogene which is co-transcribed with the downstream nad4L gene. In barley and maize, rps19 sequences are completely absent from the mitochondrion. All five of these cereals differ from rice, in which an intact, transcriptionally active mitochondrial rps19 gene is found, and this is preceded by rpl2 in an organization reminiscent of that seen in bacteria. Based on EST sequence data for maize, barley and wheat, it can be inferred that a functional rps19 gene was transferred to the nucleus prior to the divergence of the maize and rice lineages (approximately 50 million years ago), and the present-day nuclear copies encode an N-terminal sequence related to the mitochondrial targeting signal of Hsp70 (heat shock protein) in cereals. Subsequent evolutionary events have included independent losses of the mitochondrial copies in the barley and maize lineages. In the rice lineage, on the other hand, the nuclear copy was lost. This is reflected in the persistence of the mitochondrial rps19 after a period during which rps19 genes coexisted in both compartments. These observations illustrate the dynamic nature of the location and structure of genes for mitochondrial ribosomal proteins in flowering plants.