Abstract
BackgroundSterols are vital structural and regulatory components in eukaryotic cells; however, their biosynthetic pathways and functional roles in microalgae remain poorly understood.ResultsIn the oleaginous microalga Nannochloropsis oceanica, the sterol biosynthetic pathway produces phytosterols as minor products and cholesterol as the major product. The evidence together with their deduced biosynthetic pathways suggests that N. oceanica exhibits features of both higher plants and mammals. Temporal tracking of sterol profiles and sterol-biosynthetic transcripts in response to changes in light intensity and nitrogen supply reveal that sterols play roles in cell proliferation, chloroplast differentiation, and photosynthesis. Furthermore, the dynamics of fatty acid (FA) and FA-biosynthetic transcripts upon chemical inhibitor-induced sterol depletion reveal possible co-regulation of sterol production and FA synthesis, in that the squalene epoxidase inhibitor terbinafine reduces sterol content yet significantly elevates free FA production. Thus, a feedback regulation of sterol and FA homeostasis is proposed, with the 1-deoxy-D-xylulose 5-phosphate synthase (DXS, the committed enzyme in isoprenoid and sterol biosynthesis) gene potentially subject to feedback regulation by sterols.ConclusionThese findings reveal features of sterol function and biosynthesis in microalgae and suggest new genetic engineering or chemical biology approaches for enhanced oil production in microalgae.
Highlights
Sterols are vital structural and regulatory components in eukaryotic cells; their biosynthetic pathways and functional roles in microalgae remain poorly understood
The cytoplasmic mevalonic acid (MVA) pathway is the only route for biosynthesis of isopentenyl pyrophosphate (IPP), the building block for lanosterol, which is metabolized into cholesterol in animals and ergosterol in fungi [7]
In Arabidopsis, sterol biosynthetic mutants can be classified into two distinct groups: BRindependent mutants, which are defective in genes in the pathway from cycloartenol to 24-methylenelophenol [10], and BR-dependent mutants, which are defective in genes at the latter part of the sterol biosynthetic pathway [11]
Summary
Sterols are vital structural and regulatory components in eukaryotic cells; their biosynthetic pathways and functional roles in microalgae remain poorly understood. Sterols are vital components of all eukaryotic cells [1] In higher plants, they play a structural role in cell viability, embryogenesis, pattern formation, cell division, chloroplast biogenesis, and modulation of activity and distribution of membrane-bound proteins such as enzymes and receptors [2,3]. IPP can be derived via either the MVA pathway or the plastidial 1-deoxyxylulose 5-phosphate or methylerythritol phosphate (MEP) pathway, despite the former being the main contributor to sterol biosynthesis [8,9]. Besides serving as a BR precursor, sterols appear to play distinct signaling functions during plant development, since phenotypes of sterol biosynthetic mutants cannot be rescued by addition of exogenous BR [12]
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