Evidence for Regulation of Cordycepin Biosynthesis by Transcription Factors Krüppel-like factor 4 and Retinoid X receptor alpha in Cordyceps militaris

Cordyceps militaris, Chinese traditional medicinal fungus, has many bioactive properties. Cordycepin (3'-deoxyadenosine) is a major bioactive component of C. militaris. Various methods can significantly elevate cordycepin production, which suggests a diverse set of metabolic regulatory mechanisms. Thus, we aimed to identify transcription factors that regulate cordycepin biosynthesis pathways. Transcriptome analysis of wild type C. militaris, C. militaris GYS60, a cordycepin high-producing strain, and C. militaris GYS80, a low-producing strain, were used to measure expression and function of genes related to cordycepin biosynthesis. The transcriptome expression data were confirmed by qRT-PCR. We identified 155 relevant transcription factors in 19 families that included Fork head/winged helix factors, other C4 zinc finger-type factors, C2H2 zinc finger factors, tryptophan cluster factors, nuclear receptors with C4 zinc fingers, homeodomain factors, and Rel homology region factors. Energy generation and amino acid conversion pathways were activated in GYS60 so that abundance of cordycepin precursors was increased. Genes and transcription factors for rate-limiting enzymes in these pathways were identified. Overexpression of two key transcription factors, Krüppel-like factor 4 (Klf4) and Retinoid X receptor alpha (Rxra), promoted high cordycepin production in GYS60. In GYS60, Klf4 and Rxra were responsible for upregulation of genes in cordycepin biosynthesis, namely an oxidoreductase, 3',5'-cyclic AMP phosphodiesterase, a transferase, and adenylate cyclase. Upregulation of these genes increased 3'-AMP content, thereby elevating cordycepin synthesis.