Cloning, sequencing, and expression of a xylanase gene from the extreme thermophile Dictyoglomus thermophilum Rt46B.1 and activity of the enzyme on fiber-bound substrate

A genomic library of the Dictyoglomus sp. strain Rt46B.1 was constructed in the phage vector λZapII and screened for xylanase activity. A plaque expressing xylanase activity, designated B6-77, was isolated and shown to contain a genomic insert of 5.3 kb. Subcloning revealed that the xylanase activity was restricted to a internal 1,507-bp PstI-HindIII fragment which was subsequently sequenced and shown to contain a single complete open reading frame coding for a single-domain xylanase, XynA, with a putative length of 352 amino acids. Homology comparisons show that XynA is related to the family F group of xylanases. The temperature and pH optima of the recombinant enzyme were determined to be 85°C and pH 6.5, respectively. However, the enzyme was active across a broad pH range, with over 50% activity between pH 5.5 and 9.5. XynA was shown to be a true endo-acting xylanase, being capable of hydrolyzing xylan to xylotriose and xylobiose, but it could not hydrolyze xylobiose to monomeric xylose. XynA was also shown to hydrolyze xylan present in Pinus radiata kraft pulp, indicating that it may be of use as an aid in pulp bleaching. The equivalent xylanase gene was also isolated from the related bacterium Dictyoglomus thermophilum, and DNA sequencing showed these genes to be identical, which, together with the 16S small-subunit rRNA gene sequencing data, indicates that Rt46B.1 and D. thermophilum are very closely related.