Identification of initiation sites for transcription of Xenopus laevis mitochondrial DNA

The sites of transcription initiation for Xenopus laevis mtDNA have been mapped using in vitro capping and primer extension analysis of RNA isolated from oocytes. Transcription of the heavy strand initiates predominantly at a site 33 nucleotides upstream of the tRNAPhe gene. This promoter is bidirectional, with transcription of the light strand initiating only one base pair away. A second, more predominant light strand promoter is located 70 nucleotides away from the major heavy strand promoter. The overall organization of the transcription initiation sites with respect to the tRNAPhe gene and the 5' termini of major stable D-loop DNA strands resembles that of the human mtDNA genome. Analysis of the sequences surrounding the Xenopus start sites suggests that the occurrence of a conserved sequence element, ACPuTTATA, around the start sites may be important for promoter recognition and transcription initiation. This sequence is not found in human mtDNA promoters.