Identification of mitochondrial genes in Trypanosoma brucei and homology to cytochrome c oxidase II in two different reading frames.

We have determined the nucleotide sequence of a 3.3 kilobase segment of the kDNA maxicircle of Trypanosoma brucei brucei 164. The nucleotide sequence and its predicted translated sequence have homology to cytochrome c oxidase subunits I and II (CO I and II) and mammalian unidentified reading frame 1 (URF 1). Amino acid homology to CO II extends for 170 residues from the amino terminus in one reading frame and then continues in another reading frame for 39 residues to the carboxyl terminus. Similar results have been obtained for Leishmania tarentolae [de la Cruz, V.F., Neckelmann, N. and Simpson, L. (1984) J. Biol. Chem., in press] and T. brucei 427 [Hensgens, L.A.M., Brackenhoff, J., De Vries, B.F., Sloof, P., Tromp, M.C., Van Boom, J.H. and Benne, R. (1984) Nucleic Acids Res. 12, 7327-7344]. This may indicate that novel events are required for expression of this gene. Amino acid homology to URF 1 exists predominantly at the amino terminal end although no corresponding AUG codon occurs in this area. An alternative initiation codon may therefore be utilized by trypanosome mitochondria. Two other open reading frames (ORFs) were detected and these are discussed with reference to transcripts from this region. ORFs corresponding to transcripts are organized compactly and are distributed more equally on both strands compared to ORFs of other mitochondrial systems.