Self-recycling and partially conservative replication of mycobacterial methylmannose polysaccharides

The mechanism of methylmannose polysaccharide (MMP) biogenesis in nontuberculous mycobacteria (NTM) is characterized, which involves an MMP-specific hydrolase (MmpH) and a rare mannosyltransferase (ManT). MmpH and ManT structures are presented. 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 alpha-endomannosidase that hydrolyses MMP into defined-sized mannoligosaccharides that prime the elongation of new daughter MMP chains by a rare alpha-(1 -> 4)-mannosyltransferase. Therefore, MMP biogenesis occurs through a partially conservative replication mechanism, whose disruption affected mycobacterial growth rate at low temperature.

Automated summary

The mechanism of MMP biogenesis in NTM, involving MmpH and ManT, has been characterized. The structures of MmpH and ManT are presented. The increasing number of NTM infections highlights the importance of understanding their unique physiology as a health priority.