RNA editing of the tobacco mitochondrial orf38/220.

Abstract

An alloplasmic, male-sterile tobacco line obtained by sexual hybridization, resulting in introgression of the Nicotiana bigelovii cytoplasm into the Nicotiana tabacum species, is characterized by male-sterile flowers that have filaments with flat-fringed ends instead of normal stamens, and a split corolla (Burk, 1960; Chaplin, 1964). A mitochondrially encoded chimeric gene, orf38/220, corresponding to a fusion of an orf coding for 38 amino acids and the sequence coding for the carboxy-terminal 220 amino acids of the ATPase a subunit (atpA), was found to be associated with the cytoplas'mic male-sterile phenotype (P. Bergman, W. Kofer, G. Hakansson, and K. Glimelius, unpublished data). Our studies show that this chimeric gene is a target for posttranscriptional editing in which seven cytosines are replaced by uridines in the mitochondrial mRNA (Table I). At the protein level, three Pro codons (codons 180,221, and 246) were changed to Leu codons, as previously described for sugar beet atpA mRNAs (Senda et al., 1993). We have identified four additional sites of editing, codons 96 and 115 in which the Pro codon is edited to Ser, codon 142 in which Ser becomes a Leu codon, and codon 155 in which the Leu codon is transformed into a Phe codon. Eight independent clones were sequenced; three of them were fully edited, whereas in five of the clones codon 115 was unedited. Furthermore, in one of these clones codon