Alternative splicing bestows capacity for nuclear expression and mitochondrial targeting in rice
2003
Kadowaki, K. | Kubo, N.
The gene content of the plant mitochondrial genome is much larger than that of animals, insects, etc. A striking example exists in liverwort and Arabidopsis, in which 16 and 9 kinds of ribosomal protein genes have been identified, respectively, whereas none of these genes are encoded in mammals and yeast. The rice mitochondrial genome has a sequence homologous to that of the gene for ribosomal protein S14 (rps14), but the coding sequence is interrupted by internal stop codons. A functional rps14 gene was isolated from the rice nuclear genome, suggesting a gene-transfer event from the mitochondrion to the nucleus. The nuclear rps14 gene encodes a long N-terminal extension showing significant similarity to a part of mitochondrial succinate dehydrogenase subunit B (SDHB) protein from humans. Isolation of a functional rice sdhB cDNA and subsequent sequence comparison to the nucleat rps14 indicates that the 5' portions of the two cDNAs are identical. The sdhB genomic sequence shows that the SDHB-coding region is divided into two exons. Surprisingly, the RPS14-coding region is located between the two exons. DNA gel blot analysis indicates that both sdhB and rps14 are present at a single locus in the rice nucleus. These findings strongly suggest that the two gene transcripts result from a single mRNA precursor by alternative splicing. The migration of the mitochondrial rps14 sequence into the already existing sdhB gene could bestow capacity for nuclear expression and mitochondrial targeting.
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