DEVELOPMENTAL REGULATION OF TRYPANOSOMA-BRUCEI CYTOCHROME-C REDUCTASEDURING BLOOD-STREAM TO PROCYCLIC DIFFERENTIATION

The bloodstream forms of the protozoan parasite Trypanosoma brucei lac k spectrally detectable cytochromes and satisfy energy requirements ma inly by glycolysis. When infected blood is ingested by the tse-tse fly vector, the bloodstream form cells differentiate to procyclic forms t hat have fully functional mitochondria. Procyclic cells have cyanide-s ensitive, cytochrome-mediated electron transport and the full compleme nt of TCA cycle enzymes. The developmental regulation of the cytochrom e c reductase complex was examined at the RNA and protein levels. RNas e T-1 protection studies and Northern blot analyses demonstrated that bloodstream and procyclic form cells constitutively expressed the gene s for two nuclear encoded cytochrome c reductase subunits, cytochrome c(1) and subunit 4. Polyadenylated transcripts of both genes were pres ent in bloodstream form cells at up to 20% of the procyclic cell level s. These levels were significantly up-regulated sometime after the ons et of differentiation to the procyclic form. Despite the presence of s ubunit mRNAs in bloodstream form cells, subunit proteins were not dete cted until the cells had been allowed to differentiate in vitro for 6 h. Procyclic cell levels of subunit proteins and holocytochromes were reached by 48 h. Our results suggest that cytochrome c reductase is de velopmentally regulated at multiple levels, some involving post-transc riptional mechanisms.