Translation of nad9 mRNAs in mitochondria from Solanum tuberosum is restricted to completely edited transcripts

Abstract

The pool of partially and completely edited mRNAs present in plant mitochondria could potentially be translated into a mixture of divergent proteins. This possibility was investigated for the nad9 gene in potato by characterization of the mRNA population and the corresponding protein sequence. The deduced amino acid sequence of the nad9 gene product has significant similarity to the nuclear-encoded 30 kDa subunit of the bovine and Neurospora NADH:ubiquinone oxidoreductase (complex I) and to the chloroplast ndhJ gene product. Immunoprecipitation of a 27 kDa in-organello 35S labelled mitochondrial translation product with an antibody directed against the wheat nad9 gene product demonstrates its functional expression in potato and wheat. Comparison of the nad9 genomic DNA and cDNA sequences reveals seven codons to be changed by a C to U RNA-editing. Direct sequencing of RT-PCR products derived from cDNAs of different tissues of potato plants shows the presence of a significant portion of only partially edited nad9 transcripts in the various tissues. Amino acid sequencing of internal peptides of the isolated 27 kDa protein from potato tubers demonstrates homogenous translation products of only completely edited nad9 mRNAs even in the presence of partially edited mRNAs. This result suggests a pretranslational selection between edited and incompletely edited mRNAs in plant mitochondria.