Background

Echinocandin-type antifungal agents are semisynthesized from lipohexapeptides, nonribosomal peptides produced by filamentous fungi [1, 2]. As an inhibitor of β-1,3-glucan synthase, echinocandins can potently inhibit the growth of fungal pathogens by blocking the biosynthesis of β-1,3-glucan, which is a predominant and specific constituent of the cell wall in most fungi [1, 3]. Therefore, they exhibit pronounced activity against a broad range of Candida spp. and Asperillus spp., especially against azole-resistant strains [4,5,6,7]. To date, caspofungin, micafungin, and anidulafungin have been used as first-line drugs to treat invasive fungal infections [3, 6, 8, 9]. Unlike other echinocandin-type agents, micafungin exhibits excellent water solubility due to the sulfonate moiety originating from the precursor FR901379 [10, 11]. The high water solubility significantly improves the pharmacological efficacy and pharmacokinetic properties [12].

Industrial production of micafungin includes three steps: FR901379 is produced through Coleophoma empetri fermentation [11], then the palmitic acid side chain of FR901379 is deacylated and substituted with the optimized N-acyl side chain (Fig. 1) [13, 14]. The production of FR901379 is the key step in the manufacture of micafungin. However, the low titer, byproducts with similar structures, and poor mycelium pellet are obstacles to high yield during C. empetri cultivation, which increases the production cost and complicates large-scale purification processes [11, 15]. Although mutation breeding and fermentation optimization have been applied to improve the production of FR901379, the above problems remain unsolved [15, 16].

Fig. 1
figure 1

The industrial production process of micafungin and biosynthetic gene clusters of FR901379

Metabolic engineering provides promising strategies to improve the production of natural products, such as enhancing and optimizing the biosynthesis pathway [17,18,19], eliminating the competitive pathway [18,

Fig. 5
figure 5

Functional verification of self-resistance genes related to FR901379. A The transcription levels and homologous genes of the genes CEfks1 and CEfks2; MKF: FR901379 high-producing condition; LPM: FR901379 low-producing condition; FPKM: Fragments Per Kilobase of exon model per Million mapped fragments. B Schematic diagram for the construction of CEfks1 and CEfks2 disruption mutants. C FR901379 titers were quantified in the CEfks disruption strains and CEfks2 overexpression strains; WT: C. empetri MEFC09; ∆fks1: MEFC09-∆CEfks1; ∆fks2: MEFC09-∆CEfks2; OEfks2: MEFC09::CEfks2. D Images of mycelial morphology of the CEfks1 and CEfks2 disruption strains; PDA: the strains were grown on PDA medium; SM: stereo microscope images (8 × objective lens) of mycelia of the cultivation broth, all scale bars represent 2 mm; SEM: scanning electron microscopic images of CEfks disruption strains

To verify the effect of CEfks1 and CEfks2 on the production of FR901379, these two genes were individually disrupted in C. empetri MEFC09 (Fig. 5B). The double-knockout strain could not be obtained. ∆CEfks1 and ∆CEfks2 exhibited different growth rates, pellet formation, and cell wall structures (Fig. 5). Deleting CEfks1 significantly decreased the growth rate on potato dextrose agar (PDA) plates, whereas the colony of ∆CEfks2 was larger and fluffier than that of C. empetri MEFC09 (Fig. 5D). The FR901379 titer of ∆CEfks1 was 30% higher than that of C. empetri MEFC09, which might be caused by improved mycelial morphogenesis (Fig. 5C). The mycelia pellets of ∆CEfks1 were smaller and more regular than those of MEFC09. However, no obvious change was observed in the strain ∆CEfks2 (Fig. 5D). Further scanning electron microscopy (SEM) imaging revealed that the structure of the cell wall changed remarkably in both mutant strains ∆CEfks1 and ∆CEfks2. These results demonstrated that these genes both contribute to cell wall formation and affect the morphogenesis and growth of C. empetri.

To improve the production of FR901379 by enhancing product resistance, the putative self-resistance gene CEfks2 was overexpressed in C. empetri MEFC09. However, the titer of FR901379 was decreased and accompanied by a slight slowdown in growth (Fig. 5C). This result indicated that the resistance of C. empetri MEFC09 to FR901379 was sufficient to protect it from the current concentration of FR901379. Enhancing product resistance is not an effective strategy to improve production. Unlike resistance genes of other natural products, β-1,3-glucan synthase plays an important role in fungal cell wall formation. Genetic engineering of β-1,3-glucan synthase genes would affect morphogenesis and growth. Therefore, metabolic engineering based on self-resistance genes in echinocandin-producing strains will be more complicated and requires a strict balance between product resistance, growth, and mycelial morphology.

Improving the production of FR901379 by overexpresing the transcriptional activator McfJ

Overexpression of transcriptional activators is an effective strategy to increase the production of secondary metabolites [32, 33]. Overexpressing the specific transcriptional regulator lovE significantly improved the production of monacolin J in the industrial strain Aspergillus terreus [33]. However, the biosynthetic genes of FR901379 were distributed in two separate BGCs, including the core biosynthetic gene cluster mcf and the O-sulfonation gene cluster. The regulation of FR901379 biosynthesis is more ambiguous and complicated. The gene deletion and reverse transcription-polymerase chain reaction (RT‒PCR) results showed that mcfJ might be a transcriptional activator for the biosynthesis of FR901379 (Additional file 1: Fig. S3). The production of FR901379 was abolished completely in the mcfJ deletion mutant. It is consistent with the phenomenon observed in C. empetri SIPI1284, deleting Cehyp (homologous gene of mcfJ) broke the production of FR901379 [26]. The transcriptional levels of the FR901379 biosynthetic pathway, including the genes in the large mcf cluster and the separate O-sulfonation cluster, were significantly downregulated following mcfJ disruption (Fig. 6A, Additional file 1: Table S3).

Fig. 6
figure 6

Effects of overexpression of the transcriptional activator mcfJ on FR901379 production. A Heatmap of the mcf gene expression profile of C. empetri MEFC09, MEFC09-∆mcfJ, and MEFC09-J; WT: C. empetri MEFC09; J: mutant strains MEFC09-J. Relative expression levels are shown as a color gradient from low (blue) to high (red). B HPLC profiles of metabolites from C. empetri MEFC09 and MEFC09-J; 1: FR901379; 2: WF11899B; 3: WF11899C. C Titers of FR901379, WF11899B, and WF11899C were quantified in C. empetri MEFC09 and MEFC09-J in shake-flask cultures; J: mutant strains MEFC09-J; WT: C. empetri MEFC09. Statistical analysis was performed by using Student’s t test (***p < 0.001)

To further test the function of mcfJ and improve the production of FR901379, the expression cassette of mcfJ was constructed using the promoter PgpdAt and introduced into C. empetri MEFC09. The transformants harboring the expression cassette were confirmed by genomic PCR and designated MEFC09-J. The results of shake flask fermentation showed that all of the transformants produced significantly higher FR901379 titers than C. empetri MEFC09 (Fig. 6B, C). The highest FR901379 titer of 1.32 g/L was achieved in strain MEFC09-J-4, which was threefold higher than that in MEFC09 (Fig. 6C). In addition, the transcription levels of mcfJ and other biosynthetic genes of FR901379 were measured by RNA sequencing and were significantly increased in the mutant strain MEFC09-J (Fig. 6A, Additional file 1: Table S3). These results demonstrated that McfJ is indeed a transcriptional activator for the biosynthesis of FR901379. Although the biosynthetic genes were distributed in two separate BGCs, the production of FR901379 was coordinately regulated by McfJ. More importantly, because of this regulatory mechanism, overexpression of mcfJ was successfully developed as a very effective metabolic engineering strategy to increase the production capacity of FR901379.

Construction of a high-yield cell factory for FR901379 by combinatorial metabolic engineering

Overexpression of CYP enzymes mcfH and mcfF could significantly reduce the accumulation of byproducts, while overexpression of transcriptional activator mcfJ could remarkably increase the production of FR901379. Inspired by the performance of these two strategies, a mutant strain coexpressing mcfJ, mcfF and mcfH was constructed to achieve a superposition of beneficial effects. The expression cassettes of PgpdAt-mcfF-TtrpC and PgpdAt-mcfH-TtrpC were cointroduced into MEFC09-J-4 to generate the mutant strain MEFC09-JFH. The production of FR901379 was further improved in all engineered strains (Fig. 7A). The highest increase was observed in MEFC09-JFH-2, and the titer of FR901379 reached 2.03 g/L, which was 47% higher than that of MEFC09-J. In addition, the production of byproducts WF11899C was completely abolished in MEFC09-JFH-2, and the WF11899B ratio decreased from 14 to 4%. Therefore, a more efficient cell factory with a higher FR901379 titer and fewer byproducts was constructed by combinatorial metabolic engineering. This is the first transcriptional regulator identified in the biosynthesis of echinocandins, which will serve as a good reference for research on other echinocandins.

Fig. 7
figure 7

Improving FR901379 titer through combinatorial metabolic engineering and fed-batch fermentation. A Titers of FR901379, WF11899B, and WF11899C were quantified in MEFC09-JFH in shake-flask cultures. JFH: mutant strains MEFC09-JFH; J: mutant strain MEFC09-J; WT: C. empetri MEFC09. Statistical analysis was performed by using Student’s t test (**p < 0.01; ***p < 0.001). B Titers of FR901379 were quantified in the strains MEFC09, MEFC09-HF, and MEFC09-JFH cultivated in a 5 L bioreactor. C Titers of FR901379 were quantified in the strain MEFC09-JFH in fed-batch cultivation

Scale-up production of FR901379 in the 5 L bioreactor

To evaluate the scale-up potential of FR901379 production, batch experiments were carried out in a 5 L bioreactor using engineered strains MEFC09-HF and MEFC09-JFH, with MEFC09 as a control. The titers of FR901379 reached 0.9 g/L, 1.4 g/L, and 2.4 g/L in the strains MEFC09, MEFC09-HF, and MEFC09-JFH, respectively (Fig. 7B). This is significantly higher than those in the shake flask fermentation, which demonstrated the potential for production improvement in the stirred-tank bioreactor. The FR901379 productivity of MEFC09-JFH was much higher than that of MEFC09, MEFC09-HF in the batch fermentation (Additional file 1: Fig. S4). In addition, there was almost no increase in FR901379 production of MEFC09-JFH from Day 6, which was observed from Day 8 in other strains (Fig. 7B). In the fermentation medium MKF for the production of FR901379, D-sorbitol is the main carbon source. Further examination of the D-sorbitol concentration in the fermentation culture showed that D-sorbitol was completely depleted from Day 5 (Additional file 1: Fig. S5A). Therefore, fed-batch fermentation was carried out to achieve higher production of FR901379 using strain MEFC09-JFH. The downward trend in productivity has been significantly slowed. And 4.0 g/L of FR901379 was produced on Day 11 after feeding D-sorbitol three times (Fig. 7C), which was the highest production reported to date. To the best of our knowledge, it is substantially higher than the current titer achieved in industrial production .