The nucleotide sequence of a 3.2 kb segment of mitochondrial maxicircle DNA from Crithidia fasciculata containing the gene for cytochrome oxidase subunit III, the N-terminal part of the apocytochrome b gene and a possible frameshift gene; further evidence for the use of unusual initiator triplets in trypanosome mitochondria.

A 3.2 kb segment of the maxicircle of Crithidia fasciculata mitochondrial (mt) DNA contains the gene for cytochrome oxidase subunit III (coxIII), the N-terminal portion of the gene for apocytochrome b (cytb) and two partially overlapping Unassigned Reading Frames (C.URF2/1). Transcript analysis of the segment reveals that both the coxIII gene and the C.URF2/1 area are transcribed into a pair of RNA products. With the C. fasciculata gene version as a probe, a coxIII gene could not be detected in nuclear and mtDNA of Trypanosoma brucei, indicating that the cytochrome oxidases of these two closely related trypanosome species may differ. The nucleotide homology in the N-terminal region of the coxIII and cytb genes in T. brucei, Leishmania tarentolae and C. fasciculata starts at a UUA leucine codon, which adds further support to the hypothesis that apart from AUG, other initiator triplets are used in trypanosomal mitochondria: UUG, CUG and UUA, all triplets coding for leucine in the universal code. Finally, the possibility is discussed that the two overlapping URFs (C.URF2/1) in fact represent a single, frameshift containing, gene.