The nucleotide sequence and transcription of minicircular mitochondrial DNA's associated with male-fertile and cytoplasmic male-sterile lines of sugarbeet.

Male-fertile (MF) and cytoplasmic male-sterile (CMS) lines of sugarbeet possess characteristic low molecular weight minicircular mitochondrial DNA's, and Southern hybridizations revealed sequence homologies between them. The complete nucleotide sequences of minicircle a present in MF and CMS lines, and minicircle d present in some MF lines have been determined and compared to minicircle c present in all MF lines. Computer analysis revealed sequences common to all three DNA's: these may be essential to their maintenance in sugarbeet mitochondria. Analysis of the extent and arrangement of homologous sequences in minicircle a and minicircle d suggests the latter may have arisen from an intramolecular recombination event in minicircle a. The G+C contents of the minicircles differ and their possible origins are discussed. All three DNA's are transcribed. Minicircle c and minicircle d transcripts are exclusive to MF mitochondria, but are unlikely to encode protein products. Two transcripts of sufficient size and the predicted polarity to encompass a short open reading frame on minicircle a are common to both MF and CMS mitochondria.