A model for the ergosterol biosynthetic pathway in <i>Chlamydomonas reinhardtii</i>

<p>Ergosterol and 7-dehydroporiferasterol are the predominant sterols in the membranes of the alga <i>Chlamydomonas reinhardtii</i> Dangeard. Ergosterol is primarily found in most fungi, which produce their end-product sterols by way of the intermediate lanosterol. In contrast, plants rarely make ergosterol and plant sterols are made via the intermediate cycloartenol. The cycloartenol-lanosterol bifurcation has been used as a means to evolutionarily categorize species based on their sterol production. Use of bioinformatics has revealed that the green alga, <i>C. reinhardtii</i> is probably producing sterols using a pathway very similar to that of higher plants. The <i>Chlamydomonas</i> genome was searched for genes that encoded proteins exhibiting high similarity to sterol biosynthetic proteins in the higher plant <i>Arabidopsis thaliana</i> or the yeast <i>Saccharomyces cerevisiae</i>. Genes with the greatest similarity were chosen and annotated. To establish whether these genes were expressed, the presence of their transcripts was determined and quantitative RT-PCR was performed. It was observed that many of the transcripts coordinately increased in abundance after deflagellation, which induces an increase in membrane biosynthesis. This work demonstrates that <i>C. reinhardtii</i> has all of the genes necessary for the biosynthesis of ergosterol and 7-dehydroporiferasterol and that this alga uses the higher plant pathway to make a sterol normally associated with fungi. Evidence is also presented that other organisms commonly known as algae, including chlorophytes, haptophytes, rhodophytes, bacillariophytes, a phaeophyte and a cryptomonad, are likely to utilize the same biosynthetic pathway, making sterols via a cycloartenol intermediate.