Developmental regulation of trypanosome mitochondrial gene expression.

Mitochondrial respiratory activities in the protozoan parasite Trypanosoma brucei are developmentally regulated. The trypanosomes in the mammalian bloodstream derive ATP entirely from glycolysis. The trypanosomes found in the midgut of the insect vector or in culture at 26 degrees C have fully functional mitochondria with cytochrome-mediated respiration. In this paper, we show that the steady state levels of the 9 S and 12 S mitochondrial ribosomal RNAs (rRNAs) are 30-fold lower in an early developmental stage in the mammal, the slender forms, relative to the levels in the stumpy trypanosomes, a later developmental stage in the mammalian infection. Transcripts from three other mitochondrial genes, cytochrome b and subunits I and II of cytochrome oxidase, are undetectable in the slender trypanosomes and increase in the stumpy trypanosomes to levels approaching those in trypanosomes from 26 degrees C cultures. Transcription of other mitochondrial genes, including NADH-dehydrogenase subunit 5, is unregulated during trypanosome development. These results show that the level of some mitochondrial transcripts is developmentally regulated in bloodstream trypanosomes and suggest that the stumpy bloodstream trypanosomes accumulate mitochondrial transcripts prior to development of a functional mitochondrion. These results also show that the developmental activation of mitochondrial activities at 26 degrees C is not controlled at the level of mitochondrial transcription.