Unique chimeric mitochondrial genes are associated with cytoplasmic male sterility (CMS) in many higher plants. It is thought that the disruption in pollen development is a consequence of mitochondrial dysfunction associated with the chimeric genes. To analyse the effect of a chimeric mitochondrial open reading frame on mitochondrial gene expression and pollen development, we introduced the CMS- associated mitochondrial orf107 from Sorghum bicolor into the tobacco nuclear genome. Constitutive expression of orf107 fused to the yeast coxIV mitochondrial targeting presequence revealed a significant reduction of plant growth and rooting ability. A similar, but less severe phenotype was observed when orf107 was expressed without a mitochondrial targeting sequence. In contrast, the same expression strategies revealed no deleterious effect in Arabidopsis thaliana. Transient expression assays employing onion epidermal cells provide direct evidence for import of the ORF107::GFP protein without any import sequence. Our data demonstrate, that a protein of mitochondrial origin can be imported into mitochondria without the need to acquire a targeting sequence. This finding is discussed with respect to mitochondrial-nuclear gene transfer.
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