Parent Strain In Medium Research Articles

Functional analysis of the mitochondrial alternative oxidase gene (aox1) from Aspergillus niger CGMCC 10142 and its effects on citric acid production.

In this work, we constructed the aox1 disruption strains 3-4 and 4-10, as well as the aox1 overexpression strains 72 and 102 in Aspergillus niger. The energy metabolism, EMP, TCA pathways, and flux were investigated for the citric acid (CA) overproduction via the aox1 overexpression among them. As expected, the overexpression of the aox1 gene enabled a higher growth rate than that of the rate of its parent strain in medium with respiratory chain inhibitors. In liquefied corn starch medium supplemented with 0.2μg/mL antimycin A, the CA production of the overexpression strain 102 reached up to 169.1g/L, whereas the highest value of the parent strain was 158.9g/L. For the perspective of the aox1 disruption strain 4-10, the yield of CA dropped to 125.6g/L, and the loose mycelial pellets forming in the medium also revealed that the fundamentally important role of AOX in A. niger lies in the resistance to oxidative stress under fully aerobic conditions. Based on real-time qPCR gene expression analysis and measurement of intracellular ATP and NADH levels, we came to a conclusion that the higher NADH oxidation rate resulting from the overexpression of the aox1 gene mainly contributed to rate-limited step's acceleration and strengthened metabolic flow via mycelia and led to the CA yield in these strains increased by 13.5 and 10.8%, respectively. Subsequently, it was found that overexpression strains had higher AOX relative content and more oxygen consumption at different fermentation stages, which fully confirmed the close relationship between aox1 gene and energy metabolism, and comprehensively revealed aox1 gene function through the combination with the above conclusions.

Xylitol production from a mutant strain of Candida tropicalis

To characterize the kinetics of growth, sugar uptake and xylitol production in batch and fed-batch cultures for a xylitol assimilation-deficient strain of Candida tropicalis isolated via chemical mutagenesis. Chemical mutagenesis using nitrosoguanidine led to the isolation of the xylitol-assimilation deficient strain C. tropicalis SS2. Shake-flask fermentations with this mutant showed a sixfold higher xylitol yield than the parent strain in medium containing 25 g l⁻¹ glucose and 25 g l⁻¹ xylose. With 20 g l⁻¹ glycerol, replacing glucose for cell growth, and various concentrations of xylose, the studies indicated that the mutant strain resulted in xylitol yields from xylose close to theoretical. Under fully aerobic conditions, fed-batch fermentation with repeated addition of glycerol and xylose resulted in 3.3 g l⁻¹ h⁻¹ xylitol volumetric productivity with the final concentration of 220 g l⁻¹ and overall yield of 0.93 g g⁻¹ xylitol. The xylitol assimilation-deficient mutant isolated in this study showed the potential for high xylitol yield and volumetric productivity under aerobic conditions. In the evaluation of glycerol as an alternative low-cost nonfermentable carbon source, high biomass and xylitol yields under aerobic conditions were achieved; however, the increase in initial xylose concentrations resulted in a reduction in biomass yield based on glycerol consumption. This may be a consequence of the role of an active transport system in the yeast requiring increasing energy for xylose uptake and possible xylitol secretion, with little or no energy available from xylose metabolism. The study confirms the advantage of using a xylitol assimilation-deficient yeast under aerobic conditions for xylitol production with glycerol as a primary carbon source. It illustrates the potential of using the xylose stream in a biomass-based bio-refinery for the production of xylitol with further cost reductions resulting from using glycerol for yeast growth and energy production.

Actinomycin production persists in a strain of Streptomyces antibioticus lacking phenoxazinone synthase.

Truncated fragments of the phenoxazinone synthase gene, phsA, were prepared by the PCR. The resulting fragments were cloned into conjugative plasmid pKC1132 and transferred to Streptomyces antibioticus by conjugation from Escherichia coli. Two of the resulting constructs were integrated into the S. antibioticus chromosome by homologous recombination, and each of the resulting strains, designated 3720/pJSE173 and 3720/pJSE174, contained a disrupted phsA gene. Strain 3720/pJSE173 grew poorly, and Southern blotting suggested that genetic changes other than the disruption of the phsA gene might have occurred during the construction of that strain. Strain 3720/pJSE174 sporulated well and grew normally on the medium used to prepare inocula for antibiotic production. Strain 3720/pJSE174 also grew as well as the wild-type strain on antibiotic production medium containing either 1 or 5.7 mM phosphate. Strain 3720/pJSE174 was shown to be devoid of phenoxazinone synthase (PHS) activity, and PHS protein was undetectable in this strain by Western blotting. Despite the absence of detectable PHS activity, strain 3720/pJSE174 produced slightly more actinomycin than did the wild-type parent strain in medium containing 1 or 5.7 mM phosphate. The observation that strain 3720/pJSE174, lacking detectable PHS protein or enzyme activity, retained the ability to produce actinomycin supports the conclusion that PHS is not required for actinomycin biosynthesis in S. antibioticus.