A PROMOTER ELEMENT ACTIVE IN RUN-OFF TRANSCRIPTION CONTROLS THE EXPRESSION OF 2 CISTRONS OF NAD AND RPS GENES IN NICOTIANA-SYLVESTRIS MITOCHONDRIA

The expression of two mitochondrial gene clusters (orf87-nad3-nad1/A a nd orf87-nad3-rps 12) was studied in Nicotiana sylvestris. 5' and 3' t ermini of transcripts were mapped by primer extension and nuclease S1 protection. Processing and transcription initiation sites were differe ntiated by in vitro phosphorylation and capping experiments. A transcr iption initiation site, present in both gene clusters, was found 213 n ucleotides upstream of orf87. This promoter element matches the consen sus motif for dicotyledonous mitochondrial promoters and initiates run -off transcription in a pea mitochondrial purified protein fraction, P rocessing sites were identified 5' of nad3, nad1/A and rps12 respectiv ely. These results suggest that (i) the expression of the two cistrons is only controlled by one duplicated promoter element, and (ii) multi ple processing events are required to produce monocistronic nad3, nad1 /A and rps12 transcripts.