木聚糖在地球上存量丰富，仅次于纤维素类生物质材料，可作为先进生物燃料生产原料。然而，木聚糖使用中的一个主要障碍是需要从细胞壁中提取的难度大。美国能源部与联合生物能源研究所的研究人员在水稻中找到一种基因，通过抑制该基因，会提高木聚糖的提取量和蔗糖合成量。研究成果已经发表在PNAS,题目是：XAX1 from glycosyltransferase family 61 mediates Xylosyl transfer to rice xylan。
新发现的基因称为XAX1，作用是使木聚糖更易于从细胞壁中提取。JBEI的研究人员首先通过基因工程培育突变水稻，突变株敲掉了XAX1基因，发现不仅木聚糖更易于从水稻中提取，产生碳水化合物的糖化反应活性提高了60%。糖化反应活性的提高是提高先进生物燃料生产生产效率的关键。
木聚糖像纤维素一样，是植物细胞壁的主要组成部分，也是人类和动物营养的主要来源。尽管木聚糖非常重要，但是现在发现的可以合成多聚糖的酶却很少。
研究人员为了找到草本植物中木聚糖合成基因，将研究重点集中在GT61糖基转移酶家族，过去，研究人员曾通过生物信息学方法在一些草类中筛选到GT61酶。水稻是草类研究的模式生物，研究人员在水稻突变体中插入了14个基因的片段，使GT61酶在水稻突变体中高效表达，突变株命名为XAX1，因为突变导致了XAX1基因被敲出。X1A1基因的作用是在木聚糖链上添加特定的木糖等，使得木聚糖和木质素之前的链接变得更为复杂，增加木聚糖提取的难度。
随后的分析表明XAX1突变株不能合成阿魏酸、香豆酸和芳香族化合物等结合在促进木聚糖和木质素结合的阿拉伯糖上的化合物。另外，突变体还会有矮化的症状，但研究人员称已经找到一种方法避免矮化。(生物谷Bioon.com)
doi: 10.1073/pnas.1202079109
PMC:
PMID:
XAX1 from glycosyltransferase family 61 mediates xylosyltransfer to rice xylan
Dawn Chiniquya,b, Vaishali Sharmab, Alex Schultinkc,d, Edward E. Baidoob,e, Carsten Rautengartenb, Kun Chengc,d, Andrew Carrollb, Peter Ulvskovf, Jesper Harholtf, Jay D. Keaslingb,e,g, Markus Paulyc,d, Henrik V. Schellerb,c,e, and Pamela C. Ronalda,b,h,1
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 β-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 subfamily 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 (β-1,4-Xyl)4 with a β-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.
