Characterization of the potato mitochondrial transcription unit containing 'native' trnS (GCU), trnF (GAA) and trnP (UGG).
In order to identify the sequences promoting the expression of plant mitochondrial tRNA genes, we have characterized the trnS (GCU), trnF (GAA) and trnP (UGG) transcription unit of the potato mitochondrial genome. These three tRNA genes were shown to be co-transcribed as a 1800 nt long primary transcript. The transcription initiation site located 305 to 312 nt upstream of trnS is surrounded by a purine-rich region but does not contain the consensus motif proposed as a promoter element in dicotyledonous plants. Differential labelling of potato mitochondrial RNA with either guanylyltransferase or T4 polynucleotide kinase suggests that this site corresponds to the unique functional region responsible for the transcription of the three tRNA genes. The initiation site recently found upstream of Oenothera mitochondrial trnF does not seem to be used in potato mitochondria, although a very similar sequence is present 317 nt upstream of the corresponding potato gene. Major processing sites were identified at the 3' end of each tRNA gene. Another processing site, surrounded by a double hairpin structure, is located 498 nt downstream of trnP in stretch of 10 A residues. As judged from northern experiments, this region is close to the determination site of this transcription unit.