Figure 6.

Proposed sterol synthesis pathways for the different life cycle stages in C. owczarzaki. (a) In both the filopodial and aggregative stages (light orange background), cholesterol (C₂₇Δ⁵) is incorporated from the culture medium (dashed arrow) and converted into ergosterol (C₂₈Δ^5,7,22) through desaturations at positions C7(8) and C22(23), and methylation at C24 in the lateral chain. Alternative pathways are depicted, all of them leading to the same main product, ergosterol. De novo synthesis would be inhibited at the transcriptional level (figure 3b) as no squalene or cyclic sterol precursor is formed during these stages. Numbers in parentheses accompanying sterol names correspond to peak numbers in figure 4a. (b) In the cystic stage (light blue background), 7-dehydrocholesterol is produced de novo, and probably also by the Rieske C7(8)-desaturase acting on cholesterol previously incorporated by the cells before encystment, during either the filopodial or aggregative stages.