Expression of CMS-unique and flanking mitochondrial DNA sequences in Phaseolus vulgaris L

The expression of mitochondrial DNA sequences unique to a cytoplasmically male-sterile (CMS) line of Phaseolus vulgaris was investigated. RNA-blot hybridizations with strand-specific probes demonstrated CMS-unique transcripts (7.0, 6.8, 4.7, 3.3 and 2.8 kb) to be in the sense orientation with respect to the longest open reading frames within the CMS-unique region. Hybridizations revealed co-transcription of CMS-unique and upstream, atpA-coding sequences to generate the 6.8-kb RNA. However, hybridizations with CMS-unique and flanking DNA probes accounted for only 4.9 kb of the longest and most abundant (7.0 kb) CMS-unique transcript, providing indirect evidence for the involvement of a splicing process in the generation of this transcript. Sedimentation experiments demonstrated the association of 7.0- and 6.8-kb CMS-unique transcripts with polyribosomes in seedlings and floral buds of a CMS line and a line restored to fertility by the nuclear gene Fr2. However, steady-state levels of the 7.0- and 6.8-kb transcripts were decreased in the restored line relative to the CMS line.