Transportation of citrinin is regulated by the CtnC gene in the medicinal fungus Monascus purpureus. / 药用真菌紫色红曲霉通过CtnCåŸºå› è°ƒæŽ§æ¡”éœ‰ç´ çš„è½¬è¿.

Monascus is one of the most essential microbial resources in China, with thousands of years of history. Modern science has proved that Monascus can produce pigment, ergosterol, monacolin K, γ-aminobutyric acid, and other functionally active substances. Currently, Monascus is used to produce a variety of foods, health products, and pharmaceuticals, and its pigments are widely used as food additives. However, Monascus also makes a harmful polyketide component called citrinin in the fermentation process; citrinin has toxic effects on the kidneys such as teratogenicity, carcinogenicity, and mutagenicity (Gong et al., 2019). The presence of citrinin renders Monascus and its products potentially hazardous, which has led many countries to set limits and standards on citrinin content. For example, the citrinin limit is less than 0.04 mg/kg according to the Chinese document National Standard for Food Safety Food Additive Monascus (GB 1886.181-2016) (National Health and Family Planning Commission of the People's Republic of China, 2016), and the maximum level in food supplements based on rice fermented with Monascus purpureus is 100 µg/kg in the European Union (Commission of the European Union, 2019).

An oxidoreductase gene CtnD involved in citrinin biosynthesis in Monascus purpureus verified by CRISPR/Cas9 gene editing and overexpression.

Monascus produces a kind of mycotoxin, citrinin, whose synthetic pathway is still not entirely clear. The function of CtnD, a putative oxidoreductase located upstream of pksCT in the citrinin gene cluster, has not been reported. In this study, the CtnD overexpressed strain and the Cas9 constitutively expressed chassis strain were obtained by genetic transformation mediated by Agrobacterium tumefaciens. The pyrG and CtnD double gene-edited strains were then obtained by transforming the protoplasts of the Cas9 chassis strain with in vitro sgRNAs. The results showed that overexpression of CtnD resulted in significant increases in citrinin content of more than 31.7% and 67.7% in the mycelium and fermented broth, respectively. The edited CtnD caused citrinin levels to be reduced by more than 91% in the mycelium and 98% in the fermented broth, respectively. It was shown that CtnD is a key enzyme involved in citrinin biosynthesis. RNA-Seq and RT-qPCR showed that the overexpression of CtnD had no significant effect on the expression of CtnA, CtnB, CtnE, and CtnF but led to distinct changes in the expression of acyl-CoA thioesterase and two MFS transporters, which may play an unknown role in citrinin metabolism. This study is the first to report the important function of CtnD in M. purpureus through a combination of CRISPR/Cas9 editing and overexpression.