Production of Ethanol from d-Xylose by Using d-Xylose Isomerase and Yeasts

Cloning and characterization of the xyl genes from Escherichia coli

Specific xylose utilization mutants of Escherichia coli were isolated that had altered xylose isomerase ( xylA ), xylulokinase ( xylB ), and regulatory ( xylR ) or transport ( xylT ) activities. We screened the Clarke and Carbon E. coli gene bank and one clone, pLC10 -15, was found to complement the xyl mutants we had characterized. Subcloning and DNA restriction mapping allowed us to locate the xylA and xylB genes on a 1.6 kbp Bg/II fragment and a 2.6 kbp HindIII-Sa/I fragment, respectively. The identification and mapping of xyl gene promoters suggest that the xylA and xylB genes are organized as an operon having a single xylose inducible promoter preceding the xylA gene.

Involvement of oxygen and mitochondrial function in the metabolism of D-xylulose by Saccharomyces cerevisiae

Mitochondrial function associated with oxygen was required for growth of Saccharomyces cerevisiae on D-xylulose. The requirement was shown by (i) the inhibition of growth of a wild-type strain under anaerobic conditions, (ii) the inhibition of aerobic growth after treatment with inhibitors of mitochondrial function, and (iii) the lack of aerobic and anaerobic growth of nuclear and cytoplasmic petites. The mitochondrial function was associated with the channeling of catabolites of D-xylulose to growth processes, since ethanol was formed even when growth was inhibited. Mitochondrial function was implicated as well in determining the extent of growth and the concentration of ethanol in aerobic cultures of the wild-type. In such cultures, the concentration of ethanol decreased and growth increased concomitantly as aeration rate increased. A factor in this relation was considered to be the relatively poor ability of D-xylulose to inhibit the oxidative utilization of ethanol.

Amplification of D-xylose and D-glucose isomerase activities in Escherichia coli by gene cloning

A recombinant plasmid, designated pUC1002, was constructed by ligation of a HindIII restriction endonuclease fragment of Escherichia coli chromosomal DNA to vector plasmid pMB9. Strains carrying this plasmid were selected by transformation of an E. coli strain bearing the xyl-7 mutation to a xylose-positive (Xyl+) phenotype. Strains containing pUC1002 produced coordinately elevated levels of D-xylose isomerase and D-xylulose kinase. Under appropriate conditions, the isomerase also efficiently catalyzed the conversion of D-glucose to D-fructose.

Sequential utilization of mixed monosaccharides by yeasts

Four yeasts (Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida utilus, and Rhodotorula toruloides) were tested for their ability to grow and consume D-glucose, D-xylose, D-xylulose, and D-xylitol. Sequential utilization of substrates was observed when D-glucose as mixed with D-xylulose as the carbon source. Catabolite inhibition was tentatively concluded to be responsible for this regulatory mechanism. D-Glucose was also found to inhibit the utilization of D-xylose and D-xylitol in C. utilus and R. toruloides. D-Xylose, D-xylitol, and D-xylulose were consumed simultaneously by R. toruloides and C. utilus.

Enzymatic and Microbial Preparation of d -Xylulose from d -Xylose

A high- d -xylulose mixture ( d -xylose- d -xylulose = 33:67) was prepared from the cold ethanol extract of preisomerized d -xylose solution ( d -xylose- d -xylulose = 77:23). Fusarium oxysporum f. sp. lini and Aspergillus niger were demonstrated to preferentially utilize d -xylose in the mixture of d -xylose and d -xylulose. Chromatographically pure d -xylulose was thus obtained in 90% yield. A high- d -xylulose mixture was also incubated with Rhodotorula toruloides, Klebsiella pneumoniae, Candida utilis , or Mucor rouxii. d -Xylose and d -xylulose were simultaneously consumed. When borate was added to the mixture, a d -xylulose-borate complex was formed, and it could be used to protect d -xylulose from being utilized.