Fig. 3.

Heme is not needed for desaturation of fatty acids but is required for ergosterol biosynthesis in P. serpens. (A) Schematic phylogenetic tree of Δ9-fatty acid desaturases (FADS). P. serpens is the only organism that secondarily lost the cytochrome b₅ domain. The full phylogenetic tree of Δ9-fatty acid desaturase is shown in Fig. S1. (B) Analyses of fatty acid composition by gas chromatography demonstrate that in P. serpens, the desaturation of fatty acids is not affected by the absence of heme. (C) Analysis of sterol composition by TLC. Ergosterol, which is the major membrane sterol of Trypanosomatida, and lanosterol, the precursor of heme-dependent demethylation, were used as standards (S). C. fasciculata (Cf) served as a control. P. serpens synthesized a sterol that corresponded to the ergosterol standard only when heme was added to the growth medium (+H). Cells grown without heme (−H) accumulated lanosterol.