An acetylxylan esterase and a xylanase expressed from genes cloned from the ruminal fungus Neocallimastix patriciarum act synergistically to degrade acetylated xylans

A Neocallimastix patriciarum acetylxylan esterase (BnaA) was expressed from the cloned gene in Escherichia coli. Purified recombinant BnaA efficiently released acetate from soluble acetylated birchwood xylan (ABX), with a specific activity of 76 U mg(-1). In contrast, release of acetate was very inefficient from the insoluble substrates, spear grass and delignified spear grass. Addition of a recombinant xylanase, XynA, also expressed from a cloned N. patriciarum gene, had no effect on the release of acetate from ABX. However, the combination of recombinant BnaA and XynA released more acetate from spear grass and delignified spear grass than did BnaA alone. Significantly more reducing sugar was also released from all three substrates by the combination of recombinant XynA and BnaA than by XynA alone. Thus the extent of digestion of acetylated xylans by XynA appears to be limited by the acetylation. In this system BnaA does not appear to increase the rate of cleavage of insoluble substrates by XynA, but probably allows the release of shorter xylose oligomers from already solubilised acetylated xylan polymers.