Negative Regulation of Kog1 on Lipid Accumulation in the Oleaginous Fungus <i>Mucor circinelloides</i>

Oleaginous microorganisms can produce polyunsaturated fatty acids beneficial to human health through adjusting the nitrogen content in the medium. The target of rapamycin complex 1 (TORC1) is important for nitrogen sensing and then regulates lipid metabolism. However, the function of Kog1, a subunit of TORC1, in TORC1-regulated lipid metabolism in oleaginous microorganisms remains unclear. In this study, the gene <i>kog1</i> was knocked out to explore the mechanism of lipid accumulation in the oleaginous fungus <i>M. circinelloides</i> under nitrogen-limited and nitrogen-rich conditions. The results showed that the cell dry weight (CDW) of the <i>kog1</i> deletion mutant was obviously decreased from 22.2 to 15.4 g/L under nitrogen-limited conditions; however, the lipid content markedly increased by 43.2% compared to the control, from 20.8% of CDW to 29.9%. A similar trend was observed under nitrogen-rich conditions; the cell growth was significantly inhibited, the CDW was decreased from 28.6 to 23.0 g/L, and the lipid content increased by 79.6% compared to the control strain, reaching 9.7% of CDW. The addition of rapamycin further enhanced lipid accumulation in the <i>kog1</i> knockout mutant but not in the <i>tor</i> knockout mutant, indicating that Kog1 is the upstream target of rapamycin (TOR) in regulating lipid regulation. Transcriptional analysis under both nitrogen-limited and nitrogen-rich conditions notably suggested that nitrogen stress may activate Snf1/AMPK to inhibit Kog1, facilitating SREBP-1c nuclear translocation and activating fatty acid biosynthesis genes.