CNC - Center for Neuroscience and Cell Biology, University of Coimbra, 3004-504, Coimbra, Portugal.
CIBB - Center for Innovative Biomedicine and Biotechnology, University of Coimbra, Coimbra, Portugal.
Commun Biol. 2023 Jan 27;6(1):108. doi: 10.1038/s42003-023-04448-3.
The steep increase in nontuberculous mycobacteria (NTM) infections makes understanding their unique physiology an urgent health priority. NTM synthesize two polysaccharides proposed to modulate fatty acid metabolism: the ubiquitous 6-O-methylglucose lipopolysaccharide, and the 3-O-methylmannose polysaccharide (MMP) so far detected in rapidly growing mycobacteria. The recent identification of a unique MMP methyltransferase implicated the adjacent genes in MMP biosynthesis. We report a wide distribution of this gene cluster in NTM, including slowly growing mycobacteria such as Mycobacterium avium, which we reveal to produce MMP. Using a combination of MMP purification and chemoenzymatic syntheses of intermediates, we identified the biosynthetic mechanism of MMP, relying on two enzymes that we characterized biochemically and structurally: a previously undescribed α-endomannosidase that hydrolyses MMP into defined-sized mannoligosaccharides that prime the elongation of new daughter MMP chains by a rare α-(1→4)-mannosyltransferase. Therefore, MMP biogenesis occurs through a partially conservative replication mechanism, whose disruption affected mycobacterial growth rate at low temperature.
非结核分枝杆菌（NTM）感染的急剧增加使得了解其独特的生理学成为当务之急。NTM 合成两种被认为调节脂肪酸代谢的多糖：普遍存在的 6-O-甲基葡萄糖脂多糖和迄今在快速生长分枝杆菌中检测到的 3-O-甲基甘露糖多糖（MMP）。最近发现一种独特的 MMP 甲基转移酶使相邻基因参与 MMP 生物合成。我们报告了该基因簇在 NTM 中的广泛分布，包括缓慢生长的分枝杆菌，如鸟分枝杆菌，我们揭示其产生 MMP。通过 MMP 纯化和中间产物的化学酶合成组合，我们确定了 MMP 的生物合成机制，该机制依赖于我们在生化和结构上进行了表征的两种酶：一种以前未描述的α-内甘露糖苷酶，它将 MMP 水解成确定大小的甘露寡糖，为新的 MMP 链的延伸提供了条件，由一种罕见的α-（1→4）-甘露糖基转移酶来完成。因此，MMP 的生物发生是通过部分保守的复制机制进行的，该机制的破坏会影响分枝杆菌在低温下的生长速度。
