In the green alga C. reinhardtii, the predominant sterols are ergosterol and Reparixin 7-dehydroporiferasterol. These two sterols are commonly found in fungi, but not so often with higher plants. However, bioinformatics evidence supports the idea that C. reinhardtii uses the cycloartenol pathway, as genes coding for orthologs of cycloartenol cyclase and cyclopropyl isomerase, two key enzymes in the cycloartenol pathway, are found in the C. reinhardtii genome. So while C. reinhardtii synthesizes ergosterol, a sterol normally associated with the fungal biosynthetic pathway, it appears to use a pathway that more closely resembles that of higher plants. BRD73954 Earlier studies of ergosterol deficient mutants in C. reinhardtii provide evidence for the final few steps of ergosterol biosynthesis in this alga. ERG3 encodes a sterol C-5 desaturase, and belongs to the fatty acid hydroxylase superfamily. This superfamily of proteins includes C-5 sterol desaturases as well as C-4 sterol methyl oxidases. This family of proteins possesses four putative iron-binding domains. In yeast, loss of ERG3 function leads to an apparent loss of ergosterol in the membrane and an increase in the closely related precursor, episterol. The replacement of ergosterol with episterol in yeast leads to increased sensitivity to cycloheximide and an inability to grow with acetate as a sole carbon source. The efficient gene replacement techniques used in Sacchromyces cerevisiae, has allowed for the creation of mutant strains in which the ERG3 gene has been replaced with URA3, providing an experimental tool for complementation studies. Complementation of yeast knockout strains has been used successfully to identify genes involved in sterol biosynthesis in Arabidopsis. The yeast Erg25, Erg24, Erg2 and Erg3 mutants have all been complemented by the corresponding Arabidopsis genes, even though the homology between the yeast and Arabidopsis proteins was fairly low and the natural substrate in Arabidopsis differs somewhat from that of the yeast pathway.These previous results with Arabidopsis suggested that Chlamydomonas genes encoding homologous proteins might also be identified by complementation in yeast.