Transcription factor MYB15 mediates docosahexaenoic acid biosynthesis and salt tolerance in Schizochytrium sp.

Abstract

Schizochytrium sp., a unicellular heterotrophic marine protist belonging to the thraustochytrids, serves as a commercial source of docosahexaenoic acid (DHA). Enhancing strain tolerance to environmental stress promotes strain robustness and productivity. Currently, we know very little about the transcription regulators that regulate DHA biosynthesis and abiotic stress response in thraustochytrids, which hinders strain improvement by transcription factor engineering strategies. In this study, we identified a 3R-MYB transcription factor, MYB15, which was transcriptionally downregulated under low-nitrogen condition in Schizochytrium sp. ATCC20888. Deletion of the myb15 gene increased production of total lipids and DHA by 27.1% and 46.7%, respectively. Under high salt condition, myb15-deletion mutant ΔMYB15 displayed stronger salt tolerance and higher biomass than parental strain or complementation strain CMYB15. Electrophoretic mobility shift assays (EMSAs), DNase I footprinting assays, and transcriptome analysis demonstrated that MYB15 bound specifically to the 6-bp element 5′-CNGTTR-3′ in upstream regions of target genes, represses transcription of fatty acid biosynthetic genes (dga1, fabD, fas, and pfa) and salt tolerance-related genes (sos2, sos3, and betA). Taken together, we demonstrated that MYB15 acts as a critical repressor for DHA biosynthesis and salt stress response. Deletion of myb15 generates a mutant with remarkable capabilities of DHA production and salt resistance, which make it a promising Schizochytrium strain for industrial application.