The transcription of DNA in chicken mitochondria initiates from one major bidirectional promoter.

Transcription start sites of chicken mitochondrial DNA have been mapped in the control region by direct sequencing of in vitro capped mitochondrial RNA species, by primer extension and by S1 nuclease protection analysis. Transcription of the heavy strand initiates predominantly at a site 156 nucleotides upstream of the tRNA(Phe) gene, i.e. about 135 nucleotides further upstream than the corresponding sites in amphibia and mammals. On the opposite strand, transcription starts predominantly one nucleotide removed from the site in the heavy strand. The L-strand position start site is similar to that found in other vertebrates. The chicken mitochondrial DNA control region thus contains one major transcriptional promoter, whose bidirectional capacity is similar to the situation in amphibia but which contrasts to the mainly unidirectional capacity of mammalian promoters. In chicken mitochondria, the sequence comprising the start sites is A + T rich and contains an almost perfect inverted repeat which can be folded into a cruciform structure. The heavy and light strand initiation sites are flanked on their respective 3' ends by an octanucleotide sequence matching those surrounding the start sites in Xenopus laevis (5'-ACPuTTATA-3'). This motif is found associated with the H-strand start sites in mouse but is not present in human nor bovine mitochondrial DNA promoters.