Transcript mapping and processing of mitochondrial RNA in Dictyostelium discoideum

The circular mitochondrial genome of Dictyostelium discoideum has a size of 55,564 base pairs. We present here a complete and detailed transcription m ap of the mitochondrial DNA. Eight major, polycistronic transcripts encodin g polypeptides, ribosomal RNAs and interspersed transfer RNAs were identifi ed in Northern hybridization studies. Most of these polycistronic transcrip ts are subsequently processed into smaller mono-, di- or tricistronic RNAs. In some cases, the maturation involves endonucleolytic cleavage of the tra nscripts using transfer RNAs as excision signals. Primer extension experime nts mapped the 5' ends of the transcripts, which may represent transcriptio n initiation sites. Two of the polycistronic transcripts were found to be o verlapping. Based on sequence alignments of the potential transcription sta rt sites, a short oligonucleotide consensus initiation sequence has been id entified which does not reveal any significant sequence homologies to known promoter regions from other organisms.