Most of mitochondrial proteins in eukaryotes are encoded by nuclear genes. These genes seem to have been transferred from the mitochondrion to the nucleus during course of evolution. In general, the nuclear genes encode targeting signals that are involved in protein import into the mitochondria. However, very little is known about the origin and the mechanism for acquisition of the targeting signals to date. We have analyzed the content of ribosomal protein genes in the rice mitochondrial genome by using 16 kinds of ribosomal protein genes in the liverwort mitochondrial genome as probes, and found that several genes are missing from the rice mitochondrial genome. This result strongly suggests the occurrence of gene transfer events from the mitochondrion to the nucleus during flowering plant evolution. Three mitochondrial genes, which had been transferred to the nucleus (rps10, rps11 and rps14), were isolated from the rice nuclear genome and their detailed structures were analyzed. Interestingly, their 5'-coding or 5'-flanking regions showed significant similarity to those of some nuclear genes. In the case of the rps11 gene, duplication of pre-existing nuclear atpB sequence and subsequent recombination are likely to be involved in their activation. In contrast, the rps14 gene seems to have been activated by insertion of a mitochondrial sequence into a functional nuclear gene coding for mitochondrial SDHB protein. Subsequent alternative splicing event bestows functional expression of the rps14 gene. In the case of the rps10 gene, a mitochondrial-targeting signal seems to be present in the conserved region in evolution because there is no obvious N-terminal extension for the targeting signal. Accumulation of point mutations may have been involved in generation of the targeting signal for rice rps10 gene. In conclusion, utilization of pre-existing nuclear expression machinery may play an important role in the activation of mitochondrial sequences which were transferred to the nucleus.
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