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XAX1 from glycosyltransferase family 61 mediates xylosyltransfer to rice xylan

Dawn Chiniquy, Vaishali Sharma, Alex Schultink, Edward E. Baidoo, Carsten Rautengarten, Kun Cheng, Andrew Carroll, Peter Ulvskov, Jesper Harholt, Jay D. Keasling, Markus Pauly, Henrik V. Scheller and Pamela Ronald
Proceedings of the National Academy of Sciences of the United States of America
Vol. 109, No. 42 (October 16, 2012), pp. 17117-17122
Stable URL: http://www.jstor.org/stable/41763495
Page Count: 6
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XAX1 from glycosyltransferase family 61 mediates xylosyltransfer to rice xylan
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Abstract

Xylan is the second most abundant polysaccharide on Earth and represents an immense quantity of stored energy for biofuel production. Despite its importance, most of the enzymes that synthesize xylan have yet to be identified. Xylans have a backbone of ß-1,4-linked xylose residues with substitutions that include α-(1→2)-linked glucuronosyl, 4-O-methyl glucuronosyl, and α-1,2-and α-1,3-arabinofuranosyl residues. The substitutions are structurally diverse and vary by taxonomy, with grass xylan representing a unique composition distinct from dicots and other monocots. To date, no enzyme has yet been identified that is specific to grass xylan synthesis. We identified a xylose-deficient loss-of-function rice mutant in Os02g22380, a putative glycosyltransferase in a grass-specific sub-family of family GT61. We designate the mutant xax1 for xylosyl arabinosyl substitution of xylan 1. Enzymatic fingerprinting of xylan showed the specific absence in the mutant of a peak, which was isolated and determined by ¹H-NMR to be (ß-1,4-Xyl)₄ with a ß-Xylp-(1→2)-α-Araf-(1→3). Rice xax1 mutant plants are deficient in ferulic and coumaric acid, aromatic compounds known to be attached to arabinosyl residues in xylan substituted with xylosyl residues. The xax1 mutant plants exhibit an increased extractability of xylan and increased saccharification, probably reflecting a lower degree of diferulic cross-links. Activity assays with microsomes isolated from tobacco plants transiently expressing XAX1 demonstrated xylosyltransferase activity onto endogenous acceptors. Our results provide insight into grass xylan synthesis and how substitutions may be modified for increased saccharification for biofuel generation.

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