A novel strategy and kinetics analysis of half-fractional high cell density fed-batch cultivation of Zygosaccharomyces rouxii

Construction of a Zygosaccharomyces rouxii strain overexpressing the QOR gene for increased HDMF production

4-Hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) is a flavor compound widely found in natural products and is used in food as a flavor-enhancing agent. Quinone oxidoreductase (QOR) was verified as a key enzyme to synthesize HDMF in strawberry, while its impact on HDMF production by Zygosaccharomyces rouxii was still unknown. The QOR gene was cloned and overexpressed in Z. rouxii, and its impact on HDMF production by Z. rouxii was then further analyzed. At the same time, it is expected to obtain engineered strains of Z. rouxii with high HDMF production. The results showed that the engineered strains of Z. rouxii exhibit different levels of QOR gene expression and HDMF production; among them, the QOR6 strain exhibiting the highest gene expression level and HDMF production was named as ZrQOR. The HDMF production of the ZrQOR strain was significantly higher than that of wild-type Z. rouxii at 3 and 5 days of culture, with 1.41-fold and 1.08-fold increases, respectively. At 3 days of fermentation, the highest HDMF yield of ZrQOR strain was obtained (2.75 mg/L), 2 days ahead of the reported highest HDMF production by Z. rouxii. At 3, 5, and 7 days, QOR gene expression was 4.8-fold, 3.3-fold, and 5.6-fold higher in the ZrQOR strain than in the wild-type Z. rouxii, respectively. Therefore, overexpression of the QOR gene facilitates HDMF synthesis. The genetic stability of the 0-20 generation ZrQOR strain was stable, and there was no significant difference in colony shape, QOR expression, or HDMF production compared to the wild type. In this study, the genetic engineering Z. rouxii strain was used to improve HDMF production. This research has laid the groundwork for further industrial production of HDMF via microbial synthesis.

Molecular mechanisms of furanone production through the EMP and PP pathways in Zygosaccharomyces rouxii with D-fructose addition

4-Hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) and 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone (HEMF) are important aroma chemicals in fermented foods. In this study, transcriptomics, qRT-PCR and enzymology methods were used to study the molecular mechanisms of furanone production through the Embden-Meyerhof-Parnas (EMP) and Pentose Phosphate (PP) pathways in Zygosaccharomyces rouxii based on the results of our previous study. The results indicated that D-fructose addition could significantly enhance Z. rouxii biomass production. In addition, HDMF and HEMF production was increased as a result of D-fructose addition based on HPLC analysis. The significant pathways for furanone synthesis were EMP (zro00010) and PP (zro00030) based on KEGG analysis. At the mRNA level, the differentially expressed genes involved in HDMF and HEMF biosynthesis were HK, PFK1, G6PI, FBA, TPI, 6GPL, TKT, and 6PGDH. Transient overexpression of FBA and 6PGDH in Z. rouxii was significantly increased during furanone production. FBA can regulate the accumulation of dihydroxyacetone phosphate (DHAP), which is one of the precursors of HDMF, while 6PGDH can regulate the accumulation of ribulose-5-phosphate, a precursor of HEMF. In addition, the activities of PFK1, FBA, and 6PGDH were significantly correlated with furanone production. LC-MS/MS results indicated that the primary metabolites for furanone synthesis in the EMP and PP pathways gradually increased with the consumption of D-fructose. These data demonstrate that D-fructose addition can be used to generate furanones through the EMP and PP pathways in Z. rouxii.