Sterol biosynthesis by symbiotes: cytochrome P450 sterol C-22 desaturase genes from yeastlike symbiotes of rice planthoppers and anobiid beetles.

Rice planthoppers and anobiid beetles harbor intracellular yeastlike symbiotes (YLS), whose sterols are nutritionally advantageous for the host insects that cannot synthesize sterols. YLS of anobiid beetles synthesize ergosterol, whereas YLS of planthoppers produce ergosta-5,7,24(28)-trienol, which is a metabolic intermediate in the ergosterol biosynthetic pathway in yeasts. Since sterol C-22 desaturase (ERG5p, CYP61) metabolizes ergosta-5,7,24(28)-trienol into ergosta-5,7,22,24(28)-tetraenol, which is the penultimate compound in the ergosterol biosynthesis, we examined the gene of this enzyme to determine whether this enzyme works in the planthopper YLS. C-22 desaturase genes (ERG5) of YLS of the planthoppers and beetles had four introns in identical positions; such introns are not found in the reported genes of yeasts. Cytochrome P450 cysteine heme-iron ligand signature motif was well conserved among the putative amino acid sequences. The gene expression of the planthopper YLS were strongly suppressed, and the genes possessed nonsense mutations. The accumulation of ergosta-5,7,24(28)-trienol in the planthopper YLS was attributed to the inability of the planthopper YLS to produce functional ERG5p.