In order to rationally manipulate the cellular metabolism of Escherichia coli for D: -lactate production, single-gene and multiple-gene deletions with mutations in acetate kinase (ackA), phosphotransacetylase (pta), phosphoenolpyruvate synthase (pps), pyruvate formate lyase (pflB), FAD-binding D-lactate dehydrogenase (dld), pyruvate oxidase (poxB), alcohol dehydrogenase (adhE), and fumarate reductase (frdA) were tested for their effects in two-phase fermentations (aerobic growth and oxygen-limited production). Lactate yield and productivity could be improved by single-gene deletions of ackA, pta, pflB, dld, poxB, and frdA in the wild type E. coli strain but were unfavorably affected by deletions of pps and adhE. However, fermentation experiments with multiple-gene mutant strains showed that deletion of pps in addition to ackA-pta deletions had no effect on lactate production, whereas the additional deletion of adhE in E. coli B0013-050 (ackA-pta pps pflB dld poxB) increased lactate yield. Deletion of all eight genes in E. coli B0013 to produce B0013-070 (ackA-pta pps pflB dld poxB adhE frdA) increased lactate yield and productivity by twofold and reduced yields of acetate, succinate, formate, and ethanol by 95, 89, 100, and 93%, respectively. When tested in a bioreactor, E. coli B0013-070 produced 125 g/l D-lactate with an increased oxygen-limited lactate productivity of 0.61 g/g h (2.1-fold greater than E. coli B0013). These kinetic properties of D-lactate production are among the highest reported and the results have revealed which genetic manipulations improved D-lactate production by E. coli.

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Tyrosol is a phenolic compound found in olive oil and wines. The health benefits of tyrosol have attracted considerable attention. Because the tyrosol extraction from plants poses a major obstacle, biosynthesizing this compound using microbial hosts is of interest. In this study, the phenylpyruvate decarboxylase gene ARO10 and the aromatic amino acid aminotransferase gene ARO8 were introduced into Escherichia coli to generate two recombinant tyrosol producers. Deleting the prephenate dehydratase gene pheA and the phenylacetaldehyde dehydrogenase gene feaB improved the tyrosol production. Under the optimal fermentation conditions, a recombinant strain overexpressing ARO10 gene produced 4.15 mM tyrosol from 1% (w/v) glucose during a 48 h shake flask cultivation. Furthermore, when tyrosine was used as the substrate, the recombinant strain co-overexpressing ARO8 and ARO10 genes displayed a higher tyrosol yield, in which 8.71 mM tyrosol was produced from 10 mM tyrosine. This investigation suggests that microbial tyrosol production has application potential.

The diploid yeast Candida tropicalis, which can utilize n-alkane as a carbon and energy source, is an attractive strain for both physiological studies and practical applications. However, it presents some characteristics, such as rare codon usage, difficulty in sequential gene disruption, and inefficiency in foreign gene expression, that hamper strain improvement through genetic engineering. In this work, we present a simple and effective method for sequential gene disruption in C. tropicalis based on the use of an auxotrophic mutant host defective in orotidine monophosphate decarboxylase (URA3). The disruption cassette, which consists of a functional yeast URA3 gene flanked by a 0.3 kb gene disruption auxiliary sequence (gda) direct repeat derived from downstream or upstream of the URA3 gene and of homologous arms of the target gene, was constructed and introduced into the yeast genome by integrative transformation. Stable integrants were isolated by selection for Ura and identified by PCR and sequencing. The important feature of this construct, which makes it very attractive, is that recombination between the flanking direct gda repeats occurs at a high frequency (10) during mitosis. After excision of the URA3 marker, only one copy of the gda sequence remains at the recombinant locus. Thus, the resulting ura3 strain can be used again to disrupt a second allelic gene in a similar manner. In addition to this effective sequential gene disruption method, a codon-optimized green fluorescent protein-encoding gene (GFP) was functionally expressed in C. tropicalis. Thus, we propose a simple and reliable method to improve C. tropicalis by genetic manipulation.

Biodiesel has attracted considerable attention as one of the best choices among alternative and renewable fuels. Large quantities of crude glycerol are produced as a main co-product with increasing biodiesel production. Currently, the problem of waste glycerol utilization needs to be crucially addressed, not only for environmental protection but also for the economy of the biodiesel industry. In this paper, the use of crude glycerol for the production of D-lactate by engineered Escherichia coli was investigated. Engineered E. coli B0013-070 with a homolactic pathway for D-lactate synthesis by elimination of byproduct pathways (ethanol, succinate, formate and acetate) could convert 20 g L −1 of crude glycerol to 13.6 g L −1 of D-lactate with a yield of 0.67 g g −1 glycerol. Overexpression of D-lactate dehydrogenase by a low-copy vector in E. coli B0013-070 resulted in the increased production and yield of D-lactate, in which 14.5 g L −1 of D-lactate was produced with a yield of 0.72 g g −1 glycerol from crude glycerol. The effect of temperature on D-lactate fermentation by the engineered strain E. coli B0013-070-pTHldhA was also investigated, and 34 °C and 40 °C were found to be the optimal temperatures for cell growth and lactate production, respectively. The engineered strain B0013-070-pTHldhA produced 100.3 g L −1 of D-lactate with 99.97% optical purity from 531.5 g of crude glycerol with an overall productivity of 2.78 g L −1 h −1 and a yield of 75.4 g per 100 g glycerol (0.77 mol mol −1 ) using two phase fermentation combined with a temperature shifting strategy in a 7 L bioreactor. In summary, this paper shows that crude glycerol could be directly converted to D-lactate without any prior purification. † Electronic supplementary information (ESI) available. See