TRANSLATION OF NAD9 MESSENGER-RNAS IN MITOCHONDRIA FROM SOLANUM-TUBEROSUM IS RESTRICTED TO COMPLETELY EDITED TRANSCRIPTS

The pool of partially and completely edited mRNAs present in plant mit ochondria could potentially be translated into a mixture of divergent proteins, This possibility was investigated for the nad9 gene in potat o by characterization of the mRNA population and the corresponding pro tein sequence. The deduced amino acid sequence of the nad9 gene produc t has significant similarity to the nuclear-encoded 30 kDa subunit of the bovine and Neurospora NADH:ubiquinone oxidoreductase (complex I) a nd to the chloroplast ndhJ gene product. Immunoprecipitation of a 27 k Da in-organello S-35 labelled mitochondrial translation product with a n antibody directed against the wheat nad9 gene product demonstrates i ts functional expression in potato and wheat. Comparison of the nad9 g enomic 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 si gnificant portion of only partially edited nad9 transcripts in the var ious tissues. Amino acid sequencing of internal peptides of the isolat ed 27 kDa protein from potato tubers demonstrates homogenous translati on products of only completely edited nad9 mRNAs even in the presence of partially edited mRNAs. This result suggests a pretranslational sel ection between edited and incompletely edited mRNAs in plant mitochond ria.