Differential expression of two succinate dehydrogenase subunit-B genes and a transition in energy metabolism during the development of the parasitic nematode Haemonchus contortus

Abstract

The carbohydrate metabolism of free-living and parasitic stages of the sheep nematode Haemonchus contortus was studied, and it was demonstrated that during development a switch occurred from Krebs-cycle activity towards a more fermentative metabolism. During this switch a transition might take place in complex II of the respiratory chain. In the free-living (L3) and early parasitic (XL3) stages, complex II catalyses the oxidation of succinate to fumarate via the Krebs cycle, whereas in adults complex II functions in the reverse reaction, the reduction of fumarate to succinate. L3 and XL3 were shown to already possess a large anaerobic capacity. They survived well in the absence of oxygen or in the presence of cyanide, which completely blocked respiration. Krebs-cycle activity, however, was only partially inhibited by cyanide; the XL3s in particular produced in the presence of cyanide large amounts of propanol, the production of which probably functions as an alternative electron sink. For further investigation of the observed metabolic switch, complex II of the respiratory chain, a key enzyme involved in this switch, was studied. The B subunit of complex II was cloned and sequenced. These clones all showed sequences similar to the B subunit of succinate dehydrogenase from other species, and included the amino-terminal signal sequence for importation into mitochondria. Two genes were identified, types 1 and 2, based on the DNA and amino acid sequences and on the lack of cross-reaction to each other when used as probes on Southern blots. On Northern blots, the two genes showed a different expression pattern during the development of the parasite. Isotype 1 was expressed throughout the life cycle, whereas isotype 2 was expressed mainly in the free-living stages, where an aerobic metabolism prevails. This is in line with the metabolic data and is the first demonstration of differential developmental expression of two complex II subunit-B types in a eukaryote.