Sequence comparison between the flavoprotein subunit of the fumarate reductase (complex II) of the anaerobic parasitic nematode, Ascaris suum and the succinate dehydrogenase of the aerobic, free-living nematode, Caenorhabditis elegans.

Complex II in adult mitochondria of the parasitic nematode, Ascaris suum, exhibits high fumarate reductase activity and plays a key role in the anaerobic electron-transport observed in these organelles. In the present study, cDNAs for the flavoprotein (Fp) subunits of complex II have been isolated, cloned and sequenced from both A. suum and the aerobic, free-living nematode, Caenorhabditis elegans. Additional sequence at the 3' end of the mRNAs was determined by the Rapid Amplification of cDNA Ends (RACE). Nucleotide sequence analysis of the A. suum cDNAs revealed a 22-nucleotide trans-spliced leader sequence characteristic of many nematode mRNAs, an open reading frame of 1935 nucleotides and a 3' untranslated region of 616 nucleotides including a poly (A) tail from a polyadenylation signal (AATAAA). The open reading frame encoded a 645 amino acid sequence, including a 30 amino acid mitochondrial presequence. The amino acid sequences for the Fp subunits from both organisms were very similar, even though the ascarid enzyme functions physiologically as a fumarate reductase and the C. elegans enzyme a succinate dehydrogenase. The ascarid sequence was much less similar to the Escherichia coli fumarate reductase. The sensitivity of other Fp subunits to sulfhydryl reagents appears to reside in a cysteine immediately preceding a conserved arginine in the putative active site. In both nematode sequences, this cysteine is replaced by serine even though the succinate dehydrogenase activity of both enzymes is still sensitive to sulfhydryl inhibition. A cysteine six residues upstream of the serine may be involved in the sulfhydryl sensitivity of the nematode enzymes. Surprisingly, in contrast to succinate dehydrogenase activity, the fumarate reductase activity of the ascarid enzyme was not sensitive to sulfhydryl inhibition, suggesting that the mechanism of the two reactions involves separate catalytic processes.