Engineering the Xylan Utilization System in Bacillus subtilis for Production of Acidic Xylooligosaccharides

摘要

Xylans are the predominant polysaccharides in hemicelluloses and an important potential source of biofuels and chemicals. The ability of Bacillus subtilis subsp. subtilis strain 168 to utilize xylans has been ascribed to secreted glycoside hydrolase family 11 (GH11) and GH30 endoxylanases, encoded by the xynA and xynC genes, respectively. Both of these enzymes have been defined with respect to structure and function. In this study, the effects of deletion of the xynA and xynC genes, individually and in combination, were evaluated for xylan utilization and formation of acidic xylooligosaccharides. Parent strain 168 depolymerizes methylglucuronoxylans (MeGX_( n )), releasing the xylobiose and xylotriose utilized for growth and accumulating the aldouronate methylglucuronoxylotriose (MeGX_(3)) with some methylglucuronoxylotetraose (MeGX_(4)). The combined GH11 and GH30 activities process the products generated by their respective actions on MeGX_( n ) to release a maximal amount of neutral xylooligosaccharides for assimilation and growth, at the same time forming MeGX_(3) in which the internal xylose is substituted with methylglucuronate (MeG). Deletion of xynA results in the accumulation of β-1,4-xylooligosaccharides with degrees of polymerization ranging from 4 to 18 and an average degree of substitution of 1 in 7.2, each with a single MeG linked α-1,2 to the xylose penultimate to the xylose at the reducing terminus. Deletion of the xynC gene results in the accumulation of aldouronates comprised of 4 or more xylose residues in which the MeG may be linked α-1,2 to the xylose penultimate to the nonreducing xylose. These B. subtilis lines may be used for the production of acidic xylooligosaccharides with applications in human and veterinary medicine.