Discussion

There is a switch in directionality of the mitochondrial F₁F_o-ATPase between the T. brucei procyclic insect form (PCF) and slender bloodstream form (BSF). In PCF, it is thought a need for oxidative phosphorylation requires the enzyme to generate ATP. In slender BSF, the enzyme uses glycolytic ATP to drive proton pumping to maintain the essential mitochondrial membrane potential. F_o-ATPase subunit 6 is critical for proton translocation in either direction and is encoded in the mitochondrial DNA (kDNA). kDNA is therefore essential in BSF, and also in PCF, where it encodes subunits of the respiratory chain. Specific point mutations in the nuclear-encoded F_oF₁-ATPase ? subunit allow BSF survival in the absence of kDNA (Dean et al., 2013). We now show that (i) the L262Pg mutation functionally uncouples the enzyme, with dramatic consequences for F₁F_o structure and kDNA stability; (ii) kDNA⁰ cells can differentiate to transmissible stumpy forms, but these cells show a decreased lifespan, suggesting a critical role for a kDNA-encoded product; (iii) kDNA is indispensable for progression to PCF, but homozygous L262P? cells (expressing only uncoupled F₁F_o-ATPase) can still differentiate and survive in the tsetse fly midgut. Hence, despite long-held beliefs in the field, oxidative phosphorylation may not be essential in PCF in vivo. However, these PCF cells show decreased motility and cell cycle progression in vitro, a phenotype that may be caused by ATP starvation.