Terpenoid compounds, such as sterols, carotenoids or the prenyl groups of various proteins are synthesized via the mevalonate pathway. A rate-limiting step of this pathway is the conversion of 3-methylglutaryl-CoA (HMG-CoA) to mevalonic acid catalyzed by the HMG-CoA reductase. Activity of this enzyme may affect several biological processes, from the synthesis of terpenoid metabolites to the adaptation to various environmental conditions. In this study, the three HMG-CoA reductase genes (i.e. hmgR1, hmgR2 and hmgR3) of the β-carotene producing filamentous fungus, Mucor circinelloides were disrupted individually and simultaneously by a recently developed in vitro plasmid-free CRISPR-Cas9 method. Examination of the mutants revealed that the function of hmgR2 and hmgR3 are partially overlapping and involved in the general terpenoid biosynthesis. Moreover, hmgR2 seemed to have a special role in the ergosterol biosynthesis. Disruption of all three genes affected the germination ability of the spores and the sensitivity to hydrogen peroxide. Disruption of the hmgR1 gene had no effect on the ergosterol production and the sensitivity to statins but caused a reduced growth at lower temperatures. By confocal fluorescence microscopy using strains expressing GFP-tagged HmgR proteins, all three HMG-CoA reductases were localized in the endoplasmic reticulum.
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