Molecular cloning and characterization of the xylose isomerase gene from a thermophilic Bacillus species.

The gene (xylA) encoding a thermostable xylose isomerase has been isolated and characterized from a thermophilic Bacillus species for the first time. The xylA open reading frame of 1323 bp encoded a protein containing 441 amino acids with a calculated molecular weight of 50,176. The amino acid sequence of this protein showed 76% homology to xylose isomerase isolated from Bacillus subtilis and contained all the important catalytic domains of the enzyme. The gene complemented the xyl-5 mutation and produced a functional enzyme constitutively in Escherichia coli. The crude cell-free extract of E. coli recombinants exhibited xylose isomerase activity over a wide range of temperatures from 60 to 100 degrees C with an optimal enzyme activity of 10.4 Units/mg protein at 85 degrees C. This optimal temperature was one of the highest reported so far for thermostable xylose isomerases. The recombinant enzyme was found to be a tetramer with each subunit having molecular weight of 50,000.