DIFFERENTIAL EXPRESSION OF 2 SUCCINATE-DEHYDROGENASE SUBUNIT-B GENES AND A TRANSITION IN ENERGY-METABOLISM DURING THE DEVELOPMENT OF THE PARASITIC NEMATODE HAEMONCHUS-CONTORTUS

The carbohydrate metabolism of free-living and parasitic stages of the sheep nematode Haemonchus contortus was studied, and it was demonstra ted that during development a switch occurred from Krebs-cycle activit y towards a more fermentative metabolism. During this switch a transit ion might take place in complex II of the respiratory chain. In the fr ee-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 lar ge anaerobic capacity. They survived well in the absence of oxygen or in the presence of cyanide, which completely blocked respiration. Kreb s-cycle activity, however, was only partially inhibited by cyanide; th e XL3s in particular produced in the presence of cyanide large amounts of propanol, the production of which probably functions as an alterna tive electron sink. For further investigation of the observed metaboli c switch, complex II of the respiratory chain, a key enzyme involved i n 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 seque nces and on the lack of cross-reaction to each other when used as prob es on Southern blots. On Northern blots, the two genes showed a differ ent expression pattern during the development of the parasite. Isotype 1 was expressed throughout the life cycle, whereas isotype 2 was expr essed mainly in the free-living stages, where an aerobic metabolism pr evails. This is in line with the metabolic data and is the first demon stration of differential developmental expression of two complex II su bunit-B types in a eukaryote.