A mitochondrial sub-stoichiometric orf87-nad3-nad1 exonA co-transcription unit present in solanaceae was amplified in the genus Nicotiana.

Unlike other plant species, two copies of nad3 are present in Nicotiana sylvestris mitochondria. Both are localized downstream from an open reading frame (orf87 ), and are associated with either rps12 or the first exon of the nad1 gene. The orf87-nad3-nad1/A cluster is present in normal stoichiometry in Nicotiana tomentosiformis and is sub-stoichiometric in other Solanaceae, revealing recent amplification in the genus Nicotiana. It is suggested from sequence analysis that this cluster originated in an homologous recombination event that involved the nad3-rps12 intergenic region and the upstream region of an ancestral nad1 gene. Transcription patterns and RT-PCR showed that orf87-nad3-rps12 and orf87-nad3-nad1/A clusters are both co-transcription units.