Biosynthesis of Cell Envelope-Associated Phenolic Glycolipids in Mycobacterium marinum

Abstract

dPhenolic glycolipids (PGLs) are polyketide synthase-derived glycolipids unique to pathogenic mycobacteria. PGLs are found in several clinically relevant species, including various Mycobacterium tuberculosis strains, Mycobacterium leprae, and several nontuberculous mycobacterial pathogens, such as M. marinum. Multiple lines of investigation implicate PGLs in virulence, thus underscoring the relevance of a deep understanding of PGL biosynthesis. We report mutational and biochemical studies that interrogate the mechanism by which PGL biosynthetic intermediates (p-hydroxyphenylalkanoates) synthesized by the iterative polyketide synthase Pks15/1 are transferred to the noniterative polyketide synthase PpsA for acyl chain extension in M. marinum. Our findings support a model in which the transfer of the intermediates is dependent on a p-hydroxyphenylalkanoyl-AMP ligase (FadD29) acting as an intermediary between the iterative and the noniterative synthase systems. Our results also establish the p-hydroxyphenylalkanoate extension ability of PpsA, the first-acting enzyme of a multisubunit noniterative polyketide synthase system. Notably, this noniterative system is also loaded with fatty acids by a specific fatty acyl-AMP ligase (FadD26) for biosynthesis of phthiocerol dimycocerosates (PDIMs), which are nonglycosylated lipids structurally related to PGLs. To our knowledge, the partially overlapping PGL and PDIM biosynthetic pathways provide the first example of two distinct, pathwaydedicated acyl-AMP ligases loading the same type I polyketide synthase system with two alternate starter units to produce two structurally different families of metabolites. The studies reported here advance our understanding of the biosynthesis of an important group of mycobacterial glycolipids. Mycobacterial infections are responsible for devastating morbidity and mortality worldwide (1-5). A critical player in the ability of the mycobacteria to produce disease is a formidable cell envelope believed to be responsible for the intrinsic resilience of the mycobacteria to inhospitable environments, antimicrobial agents, and host immune defenses (6-12). Among the unique components found in the cell envelope of several pathogenic mycobacteria are two structurally related families of glycosylated and nonglycosylated lipids commonly referred to as phenolic glycolipids (PGLs) and phthiocerol dimycocerosates (PDIMs), respectively (for a review, see reference 13). PGLs and PDIMs have unusual lipid scaffolds consisting of ␤-diol-containing, long-chain, aliphatic polyketides esterified with long-chain, multimethylbranched fatty acids onto the diol functionality (Fig. 1). PGLs and PDIMs are found in several mycobacterial pathogens (e.g., Mycobacterium tuberculosis strains, Mycobacterium bovis, Mycobacterium leprae, and Mycobacterium marinum) and thought to be constituents of the characteristic mycobacterial outer membrane. Multiple lines of investigation have provided considerable support for the idea that PGLs and PDIMs are implicated in virulence via co...

System-wide coordinates of higher order functions in host-pathogen environment upon Mycobacterium tuberculosis infection

“…They participate in cholesterol degradation pathway and play an important role in intracellular survival of Mtb by providing cholesterol as an alternative carbon source 54 – 59 . Another two operons pks1-pks15-fad22-fad29 and fadD26-ppsABCDE-drrABC were involved in synthesis of phthiodiolone dimycocerosates (DIMs) and immuno-modulatory phenolic glycolipids, which have a role in immuno-modulation 60 – 62 . Some of the genes ( drrA , drrB and drrC ) in aforementioned operon were known to encode for a drug efflux pump, which confers resistance to many antibiotics 63 .…”

High-Throughput Screen for Cell Wall Synthesis Network Module in Mycobacterium tuberculosis Based on Integrated Bioinformatics Strategy

“…However, the genes contained in Subnet 4, such as ppsa and ppsb, were significantly altered in drug-resistant bacteria (Cantrell et al, 2013). We believe that ppsa changes the expression of PDIM in the cell wall by changing the approach of the multi-subunit non-iterated polyketide synthase system (Vergnolle et al, 2015). This makes the bacterial cell wall thicker and causes bacterial drug efflux.…”

Revisiting the expression signature of pks15/1 unveils regulatory patterns controlling phenolphtiocerol and phenolglycolipid production in pathogenic mycobacteria

“…Production of PGL's phenolphthiocerol moiety starts with the enzyme encoded by Rv2949c that catalyses the formation of p-hydroxybenzoic acid (p-HBA) that will later be activated by the fadD22 product, with p-hydroxybenzoyl-AMP ligase activity [4], and finally elongated with malonyl-CoA as extender unit by pks15/1, in a reaction that may comprise eight to nine elongation cycles [11]. The product of fadD29, a fatty acyl-AMP ligase, is then responsible for activation of p-hydroxyphenylalkanoates, later transferred onto the ppsA product and, finally, elongated with malonyl-CoA and mehtylmalonyl-CoA by PpsB-PpsE to yield the phenolphtiocerol moiety of PGL [14][15][16]. As described [17], upon the entrance of Mtb into its target cells, a cascade of events is triggered by the immune system that, in an immunocompetent host, leads to granuloma formation and bacterial confinement.…”