Transcriptional response of Tetrahymena thermophila to exogenous cholesterol

The model ciliate Tetrahymena thermophila satisfies its growth using triterpenoid alcohols, mainly tetrahymanol, as a surrogate of sterols. When sterols are present in the environment, T. thermophila efficiently incorporates and modifies them. T. thermophila can modify exogenous sterols by desaturation at positions C5(6), C7(8) and C22(23), and also by de-ethylation of C24. Three out of four of the enzymes involved in the sterol modification pathway were previously described by our group. However, identification of the sterol C22 desaturase remained elusive, as well as other basic aspects of this metabolism. To get more insights into this peculiar metabolism we here perform a whole transcriptome analysis of T. thermophila in response to exogenous cholesterol. We found 356 T. thermophila genes to be differentially expressed after supplementation with cholesterol for two hours. Among those genes that were upregulated under our experimental conditions, we identified two genes belonging to the long-spaced family of desaturases that we propose as putative sterol C22 desaturases. Moreover, we determined that the inhibition of tetrahymanol synthesis after sensing exogenous cholesterol occurs by a transcriptional downregulation of genes involved in squalene synthesis and cyclization. Finally, we also identified several uncharacterized genes that can be putatively involved in sterols transport and signaling. mRNA profiles of T. thermophila strain CU428 (wild type) cultivated with (TREATED) and without (CONTROL) the addition of cholesterol to the growth media, by triplicate.