Novel Type III Polyketide Synthases Biosynthesize Methylated Polyketides in Mycobacterium marinum.

Mycobacterial pathogenesis is hallmarked by lipidic polyketides that decorate the cell envelope and mediate infection. However, factors mediating persistence remain largely unknown. Dynamic cell wall remodeling could facilitate the different pathogenic phases. Recent studies have implicated type III polyketide synthases (PKSs) in cell wall alterations in several bacteria. Comparative genome analysis revealed several type III pks gene clusters in mycobacteria. In this study, we report the functional characterization of two novel type III PKSs, MMAR_2470 and MMAR_2474, in Mycobacterium marinum. These type III pkss belong to a unique pks genomic cluster conserved exclusively in pathogenic mycobacteria. Cell-free reconstitution assays and high-resolution mass spectrometric analyses revealed methylated polyketide products in independent reactions of both proteins. MMAR_2474 protein exceptionally biosynthesized methylated alkyl-resorcinol and methylated acyl-phloroglucinol products from the same catalytic core. Structure-based homology modeling, product docking, and mutational studies identified residues that could facilitate the distinctive catalysis of these proteins. Functional investigations in heterologous mycobacterial strain implicated MMAR_2474 protein to be vital for mycobacterial survival in stationary biofilms. Our investigations provide new insights into type III PKSs conserved in pathogenic mycobacterial species.