Expression of the mitochondrial genome in Xenopus laevis: a map of transcripts.

The twelve most abundant transcripts in X. laevis mitochondria were characterized, and their coding regions were mapped on the mitochondrial DNA (mtDNA) by R loop mapping in the electron microscope and by RNA gel transfer hybridization. The transcripts map in nonoverlapping positions with one exception, and they account for about 80% of the coding capacity of mtDNA. Ten of the twelve RNA molecules contain poly(A): the two poly(A)-lacking transcripts are the rRNAs. Analysis withe single-strand-specific nucleases clearly demonstrated the absence of intervening sequences from the coding regions for seven RNAs. For two RNAs, uninterrupted coding sequences are strongly suggested and one RNA could not be analyzed. Eight transcripts of low abundance and high molecular weight were characterized, and their coding regions were mapped approximately. They overlap the coding regions of the abundant mitochondrial RNAs and could be precursors of these RNAs. Most of the RNA molecules characterized were shown to be transcribed from the heavy strand of mtDNA. No abundant discrete light-strand transcript was found.