Characterization of Ruminiclostridium josui arabinoxylan arabinofuranohydrolase, RjAxh43B, and RjAxh43B-containing xylanolytic complex.

A novel gene (axh43B) from Ruminiclostridium josui encoding a cellulosomal enzyme consisting of a catalytic module of subfamily GH43_10, a family-6 carbohydrate-binding module, and a dockerin module, was expressed using Escherichia coli. RjAxh43B released only arabinose from arabinoxylan and 2,3-di-α-l-arabinofuranosyl xylotriose, but not 3-α-l-arabinofuranosyl xylobiose or 2-α-l-arabinofuranosyl xylotriose, strongly suggesting that RjAxh43B is an arabinoxylan α-l-1,3-arabinofuranohydrolase capable of cleaving α-1,3-linked arabinose residues of doubly arabinosylated xylan. When Axh43B was mixed with the recombinant scaffolding protein RjCipA of R. josui at a molar ratio of 6:1, the activity of the RjAxh43B-RjCipA complex (6:1) toward insoluble wheat arabinoxylan was similar to that of RjAxh43B alone, suggesting that RjAxh43B does not show a proximity effect, which is defined as an activity enhancement effect caused by the presence of plural catalytic subunits adjoining each other. When RjAxh43A was mixed with xylanase RjXyn10C, they acted synergistically toward insoluble wheat arabinoxylan and rice straw powder in the absence of RjCipA. Furthermore, the RjAxh43B-RjXyn10C-RjCipA (3:3:3) complex had higher activity toward insoluble wheat arabinoxylan than a mixture of RjAxh43B and RjXyn10C without RjCipA, suggesting that incorporation of a xylanase and an α-l-arabinofuranosidase into a cellulosome is beneficial for more efficiently degrading arabinoxylan.