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Abstract
BackgroundProduction of L-sorbose from D-sorbitol by Gluconobacter oxydans is the first step to produce L-ascorbic acid on industrial scale. The sldhAB gene, which encodes the sorbitol dehydrogenase (SLDH), was overexpressed in an industrial strain G. oxydans WSH-003 with a strong promoter, PtufB. To enhance the mRNA abundance, a series of artificial poly(A/T) tails were added to the 3′-terminal of sldhAB gene. Besides, their role in sldhAB overexpression and their subsequent effects on L-sorbose production were investigated.ResultsThe mRNA abundance of the sldhAB gene could be enhanced in G. oxydans by suitable poly(A/T) tails. By self-overexpressing the sldhAB gene in G. oxydans WSH-003 with an optimal poly(A/T) tail under the constitutive promoter PtufB, the titer and the productivity of L-sorbose were enhanced by 36.3% and 25.0%, respectively, in a 1-L fermenter. Immobilization of G. oxydans-sldhAB6 cells further improved the L-sorbose titer by 33.7% after 20 days of semi-continuous fed-batch fermentation.ConclusionsThe artificial poly(A/T) tails could significantly enhance the mRNA abundance of the sldhAB. Immobilized G. oxydans-sldhAB6 cells could further enlarge the positive effect caused by enhanced mRNA abundance of the sldhAB.
Highlights
Production of L-sorbose from D-sorbitol by Gluconobacter oxydans is the first step to produce L-ascorbic acid on industrial scale
Results and discussion Quantitative real-time PCR (qRT-PCR) analysis of the sldhAB gene In order to identify the role of the poly(A/T) tails in sldhAB expression, qRT-PCR analysis was performed with different engineered G. oxydans strains grown on D-sorbitol to the beginning of the stationary phase
This result showed that the mRNA stability and transcription of the sldhAB gene in G. oxydans could be markedly enhanced by suitable poly(A/T) tails
Summary
Production of L-sorbose from D-sorbitol by Gluconobacter oxydans is the first step to produce L-ascorbic acid on industrial scale. Gluconobacter oxydans is widely used in biotransformation due to its ability to incompletely oxidize D-sorbitol, glycerol, and glucose to L-sorbose [5], dihydroxypropanone [6], and gluconic acid [7], respectively. These oxidation reactions are performed by membrane-bound dehydrogenases located on the outer surface of the cytoplasmic membrane, whereas the oxidation products accumulate in the culture medium. This indicated that an engineered G. oxydans strain that overexpresses the sldhAB gene could enhance the L-sorbose production
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