Editing and translation of ribosomal protein S13 transcripts: Unedited translation products are not detectable in maize mitochondria

Maize mitochondrial transcripts for the ribosomal protein S13 gene (rps13) have six C- to -U editing sites, and each nucleotide conversion causes a change in the amino acid specified by the effected codons. Sequence analysis of 30 cDNA clones indicated that 73% of the cDNAS were edited at all six sites and 3% were completely unedited. Antibodies were produced against synthetic peptides that corresponded to unedited or edited translation products at editing sites V and VI (80% and 83% edited, respectively). Antibody preparations were purified that selectively recognized the edited or unedited forms of the epitope. The antibody preparations were highly sensitive to the amino-acid residue encoded at editing site VI, but relatively insensitive to the residue encoded at editing site V. Immunological analyses demonstrated that the edited translation product accumulated as a ribosomal protein, but that the unedited translation product was not detected in the mitochondrion, in the ribosomal fraction, or in a post-ribosomal supernatant. These results, taken together with other studies which demonstrated that incompletely edited transcripts are incorporated into polyribosomes, suggest that incompletely edited transcripts may be translated, but polypeptides encoded by incompletely edited RNAs may be unstable and, consequently, fail to accumulate.