Smith James Amor, Bar-Peled Maor
Complex Carbohydrate Research Center (CCRC), University of Georgia, Athens, GA, United States of America.
Dept. of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States of America.
PLoS One. 2017 Sep 20;12(9):e0184953. doi: 10.1371/journal.pone.0184953. eCollection 2017.
The branched-chain sugar apiose was widely assumed to be synthesized only by plant species. In plants, apiose-containing polysaccharides are found in vascularized plant cell walls as the pectic polymers rhamnogalacturonan II and apiogalacturonan. Apiosylated secondary metabolites are also common in many plant species including ancestral avascular bryophytes and green algae. Apiosyl-residues have not been documented in bacteria. In a screen for new bacterial glycan structures, we detected small amounts of apiose in methanolic extracts of the aerobic phototroph Geminicoccus roseus and the pathogenic soil-dwelling bacteria Xanthomonas pisi. Apiose was also present in the cell pellet of X. pisi. Examination of these bacterial genomes uncovered genes with relatively low protein homology to plant UDP-apiose/UDP-xylose synthase (UAS). Phylogenetic analysis revealed that these bacterial UAS-like homologs belong in a clade distinct to UAS and separated from other nucleotide sugar biosynthetic enzymes. Recombinant expression of three bacterial UAS-like proteins demonstrates that they actively convert UDP-glucuronic acid to UDP-apiose and UDP-xylose. Both UDP-apiose and UDP-xylose were detectable in cell cultures of G. roseus and X. pisi. We could not, however, definitively identify the apiosides made by these bacteria, but the detection of apiosides coupled with the in vivo transcription of bUAS and production of UDP-apiose clearly demonstrate that these microbes have evolved the ability to incorporate apiose into glycans during their lifecycles. While this is the first report to describe enzymes for the formation of activated apiose in bacteria, the advantage of synthesizing apiose-containing glycans in bacteria remains unknown. The characteristics of bUAS and its products are discussed.
支链糖芹糖长期以来被广泛认为仅由植物物种合成。在植物中，含芹糖的多糖存在于维管束植物细胞壁中，为果胶聚合物鼠李半乳糖醛酸聚糖II和芹糖半乳糖醛酸聚糖。含芹糖基的次生代谢产物在许多植物物种中也很常见，包括原始的无维管束苔藓植物和绿藻。尚未在细菌中记录到芹糖基残基。在一项对新细菌聚糖结构的筛选中，我们在需氧光合细菌玫瑰双球菌和致病土壤细菌豌豆黄单胞菌的甲醇提取物中检测到少量芹糖。芹糖也存在于豌豆黄单胞菌的细胞沉淀中。对这些细菌基因组的研究发现了与植物UDP-芹糖/UDP-木糖合酶（UAS）蛋白质同源性相对较低的基因。系统发育分析表明，这些细菌类UAS同源物属于一个与UAS不同的进化枝，并且与其他核苷酸糖生物合成酶分开。三种细菌类UAS蛋白的重组表达表明，它们能将UDP-葡萄糖醛酸积极转化为UDP-芹糖和UDP-木糖。在玫瑰双球菌和豌豆黄单胞菌的细胞培养物中均可检测到UDP-芹糖和UDP-木糖。然而，我们无法明确鉴定这些细菌产生的芹糖苷，但芹糖苷的检测以及类UAS的体内转录和UDP-芹糖的产生清楚地表明，这些微生物在其生命周期中已经进化出将芹糖纳入聚糖的能力。虽然这是第一份描述细菌中活化芹糖形成酶的报告，但在细菌中合成含芹糖聚糖的优势仍然未知。本文讨论了类UAS及其产物的特性。
