Kog1 Represses Lipid Accumulation in Mucor circinelloides: A Transcriptomic Analysis Across Nitrogen Conditions

Oleaginous microorganisms usually accumulate large amounts of lipids under nitrogen limitation and in a carbon-abundant environment. However, how cells sense changes in nitrogen and carbon levels in the culture medium remains a research hotspot. Previous studies have found that the target of rapamycin complex 1 (TORC1) plays a core role in lipid accumulation in oleaginous microorganisms. The results of the Kog1 (the member proteins of TORC1) knockout strain constructed earlier by our group showed that the Kog1 negatively regulated lipid accumulation in the oleaginous fungus Mucor circinelloides. In this study, transcriptomic analysis of the knockout and control strains under nitrogen-limited and nitrogen-sufficient culture was carried out to investigate significant differences in lipid accumulation. Kog1 knockout led to a significant decrease in cell dry weight and an increase in lipid content in M. circinelloides. The transcriptomic results showed that genes encoding the glyoxylic acid cycle and genes encoding acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and Δ9 desaturase in lipid synthesis were upregulated to varying degrees under both conditions, indicating enhanced lipid metabolism that ultimately led to increased lipid accumulation. The knockout of the Kog1 gene also activated the pyruvate-acetaldehyde-acetate metabolic axis and significantly modified the branched-chain amino acid metabolic network, suggesting that Kog1 knockout reprograms the pathway of branched-chain amino acid synthesis and degradation, shifting the carbon flux from amino acid metabolism to acetyl-CoA accumulation. In addition, the gene encoding the SSK1p transcription factor, which participates in the nutrient stress response, was upregulated 41.9- and 51.9-fold in the Kog1 knockout strain compared with the control strain under nitrogen-limited and nitrogen-sufficient conditions, respectively.