A promoter element active in run-off transcription controls the expression of two cistrons of nad and rps genes in Nicotiana sylvestris mitochondria

The expression of two mitochondrial gene clusters (orf87-nad3-nad1/A and orf87-nad3-rps12) was studied in Nicotiana sylvestris. 5' and 3' termini of transcripts were mapped by primer extension and nuclease S1 protection. Processing and transcription initiation sites were differentiated by in vitro phosphorylation and capping experiments. A transcription initiation site, present in both gene clusters, was found 213 nucleotides upstream of orf87. This promoter element matches the consensus motif for dicotyledonous mitochondrial promoters and initiates run-off transcription in a pea mitochondrial purified protein fraction. Processing sites were identified 5' of nad3, nad1/A and rps12 respectively. These results suggest that (i) the expression of the two cistrons is only controlled by one duplicated promoter element, and (ii) multiple processing events are required to produce monocistronic nad3, nad1/A and rps12 transcripts.