Sterol Biosynthesis in Four Green Algae: A Bioinformatic Analysis of the Ergosterol Versus Phytosterol Decision Point

Authors

Adam Voshall, Harvard Medical SchoolFollow
Nakeirah T.M. Christie, Yale University
Suzanne L. Rose, University of Nebraska-LincolnFollow
Maya Khasin, USDA, Lincoln, NebraskaFollow
James L. Van Etten, University of Nebraska - LincolnFollow
Jennifer E. Markham, University of Nebraska-LincolnFollow
Wayne R. Riekhof, University of Nebraska–LincolnFollow
Kenneth Nickerson, University of Nebraska - LincolnFollow

ORCID IDs

Adam Voshall https://orcid.org/0000-0003-1606-4597

Jennifer E. Markham 0000-0003-2397-3131

Date of this Version

2021

Citation

Journal of Phycology (2021) 57: 1,199–1,211

doi: 10.1111/jpy.13164-20-064

Comments

Copyright 2021, the authors

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License (CC BY-NC)

Abstract

Animals and fungi produce cholesterol and ergosterol, respectively, while plants produce the phytosterols stigmasterol, campesterol, and bsitosterol in various combinations. The recent sequencing of many algal genomes allows the detailed reconstruction of the sterol metabolic pathways. Here, we characterized sterol synthesis in two sequenced Chlorella spp., the free-living C. sorokiniana, and symbiotic C. variabilis NC64A. Chlamydomonas reinhardtii was included as an internal control and Coccomyxa subellipsoidea as a plant-like outlier. We found that ergosterol was the major sterol produced by Chlorella spp. and C. reinhardtii, while C. subellipsoidea produced the three phytosterols found in plants. In silico analysis of the C. variabilis NC64A, C. sorokiniana, and C. subellipsoidea genomes identified 22 homologs of sterol biosynthetic genes from Arabidopsis thaliana, Saccharomyces cerevisiae, and C. reinhardtii. The presence of CAS1, CPI1, and HYD1 in the four algal genomes suggests the higher plant cycloartenol branch for sterol biosynthesis, confirming that algae and fungi use different pathways for ergosterol synthesis. Phylogenetic analysis for 40 oxidosqualene cyclases (OSCs) showed that the nine algal OSCs clustered with the cycloartenol cyclases, rather than the lanosterol cyclases, with the OSC for C. subellipsoidea positioned in between the higher plants and the eight other algae. With regard to why C. subellipsoidea produced phytosterols instead of ergosterol, we identified 22 differentially conserved positions where C. subellipsoidea CAS and A. thaliana CAS1 have one amino acid while the three ergosterol producing algae have another. Together, these results emphasize the position of the unicellular algae as an evolutionary transition point for sterols.