The Aspergillus niger D-xylulose kinase gene is co-expressed with genes encoding arabinan degrading enzymes, and is essential for growth on D-xylose and L-arabinose.

The Aspergillus niger D-xylulose kinase encoding gene has been cloned by complementation of a strain deficient in D-xylulose kinase activity. Expression of xkiA was observed in the presence of L-arabinose, L-arabitol and D-xylose. Expression of xkiA is not mediated by XLNR, the xylose-dependent positively-acting xylanolytic regulator. Although the expression of xkiA is subject to carbon catabolite repression, the wide domain regulator CREA is not directly involved. The A. niger D-xylulose kinase was purified to homogeneity, and the molecular mass determined using electrospray ionization mass spectrometry agreed with the calculated molecular mass of 62816.6 Da. The activity of XKIA is highly specific for D-xylulose. Kinetic parameters were determined as Km(D-xylulose) = 0.76 mM and Km(ATP) = 0.061 mM. Increased transcript levels of the genes encoding arabinan and xylan degrading enzymes, observed in the xylulose kinase deficient strain, correlate with increased accumulation of L-arabitol and xylitol, respectively. This result supports the suggestion that L-arabitol may be the specific low molecular mass inducer of the genes involved in arabinan degradation. It also suggests a possible role for xylitol in the induction of xylanolytic genes. Conversely, overproduction of XKIA did not reduce the size of the intracellular arabitol and xylitol pools, and therefore had no effect on expression of genes encoding xylan and arabinan degrading enzymes nor on the activity of the enzymes of the catabolic pathway.