Cloning and characterization of a novel NAD+-dependent glyceraldehyde-3-phosphate dehydrogenase gene fromCandida glycerinogenesand use of its promoter

Toxicants improve glycerol production in the fermentation of undetoxified hydrolysate by Candida glycerinogenes

OBJECTIVES

Toxicants inhibit microbial fermentation and reduce product titres. This work investigated the glycerol production characteristics of Candida glycerinogenes in highly toxic unwashed undetoxified hydrolysate and provided new ideas for high glycerol production from hydrolysates.

RESULTS

The unwashed hydrolysate contains higher concentrations of toxicants, such as furfural, acetic acid, phenols and NaCl than the washed alkali-treated bagasse hydrolysate. C. glycerinogenes fermented unwashed undetoxified hydrolysate yielded 36.1 g/L glycerol, 15.8% higher than the washed hydrolysate, suggesting that the toxicants stimulated glycerol synthesis. qRT-PCR analysis showed that toxicants of unwashed undetoxified hydrolysates greatly up-regulated the transcript levels of the genes GPD1, HXT4 and MSN4 et al. Overexpressing the above genes increased glycerol production by 27.9% to 46.1 g/L. And it was further increased by 8.8% to 50.1 g/L in a 5 L bioreactor.

CONCLUSIONS

This result proves that toxicants in lignocellulosic hydrolysates can increase the titre of microbial glycerol production.

Tuning Geraniol Biosynthesis via a Novel Decane-Responsive Promoter in Candida glycerinogenes

Geraniol is a rose-scented monoterpene with significant commercial and industrial value in medicine, condiments, cosmetics, and bioenergy. Here, we first targeted geraniol as a reporter metabolite and explored the suitability and potential of Candida glycerinogenes as a heterologous host for monoterpenoid production. Subsequently, dual-pathway engineering was employed to improve the production of geraniol with a geraniol titer of 858.4 mg/L. We then applied a synthetic hybrid promoter approach to develop a decane-responsive hybrid promoter based on the native promoter P_GAP derived from C. glycerinogenes itself. The hybrid promoter was able to be induced by n-decane with 3.6 times higher transcriptional intensity than the natural promoter P_GAP. In particular, the hybrid promoter effectively reduces the conflict between cell growth and product formation in the production of geraniol. Ultimately, 1194.6 mg/L geraniol was obtained at the shake flask level. The strong and tunable decane-responsive hybrid promoter developed in this study provides an important tool for fine regulation of toxic terpenoid production in cells.

A synthetic hybrid promoter for D-xylonate production at low pH in the tolerant yeast Candida glycerinogenes

The tolerant yeast Candida glycerinogenes, with high D-xylonate and low-pH tolerances, was used as the host for D-xylonate production at low pH in this study. A low-pH inducible promoter, pGUKd, was engineered using the core promoter of the glyceraldehyde-3-phosphate dehydrogenase gene (pGAP) combined with the upstream activating sequence of the promoter of the guanylate kinase gene (pGUK1) that had substituted pH-responsive TF binding sites. The recombinant cells that expressed GFP from the hybrid promoter pGUKd displayed dramatically increased fluorescence intensity at pH 2.5, thus verifying that pGUKd is a low-pH inducible promoter. The promoter pGUKd was then used to express the D-xylose dehydrogenase gene xylB, resulting in increased expression levels of xylB at low pH. The recombinant protein exhibited higher specific activities under lower pH conditions and produced 38 g/l D-xylonate at pH 2.5. This rate is much higher than that produced by fermentation at pH 5.5. These results suggest that the novel hybrid promoter pGUKd functions to direct the production of D-xylonate at low pH, and we provide a candidate genetic tool for the stress tolerant yeast C. glycerinogenes.