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 co mplementation of a strain deficient in D-xylulose kinase activity. Expressi on of xkiA was observed in the presence Of L-arabinose, L-arabitol and D-xy lose. Expression of xkiA is not mediated by XLNR, the xylose-dependent posi tively-acting xylanolytic regulator. Although the expression of xkiA is sub ject to carbon catabolite repression, the wide domain regulator CREA is not directly involved. The A. niger D-xylulose kinase was purified to homogene ity, and the molecular mass determined using electrospray ionization mass s pectrometry agreed with the calculated molecular mass of 62816.6 Da. The ac tivity of XKIA is highly specific for D-xylulose. Kinetic parameters were d etermined as K-m(D-xylulose) = 0.76 mm and K-m(ATP) = 0.061 mm. Increased t ranscript levels of the genes encoding arabinan and xylan degrading enzymes , observed in the xylulose kinase deficient strain, correlate with increase d accumulation of L-arabitol and xylitol, respectively. This result support s the suggestion that L-arabitol may be the specific low molecular mass ind ucer of the genes involved in arabinan degradation. It also suggests a poss ible role for xylitol in the induction of xylanolytic genes. Conversely, ov erproduction of XKIA did not reduce the size of the intracellular arabitol and xylitol pools, and therefore had no effect on expression of genes encod ing xylan and arabinan degrading enzymes nor on the activity of the enzymes of the catabolic pathway.