INSIGHTS INTO THE REACTIVATION, REGULATION AND ESSENTIALITY OF OXIDATIVE PROTEIN FOLDING PATHWAYS IN ACTINOBACTERIA

Accurate disulfide bond formation is important for proper folding, stability and function of exported proteins. The process of disulfide bond formation, termed oxidative protein folding, is catalyzed by thiol-disulfide oxidoreductase enzymes. Oxidative protein folding pathways influence processes essential for bacterial physiology and pathogenicity. In the Gram-positive actinobacterial pathogens Actinomyces oris and Corynebacterium diphtheriae oxidative protein folding is catalyzed by the primary thiol-disulfide oxidoreductase MdbA. MdbA is required for assembly of adhesive pilus, which mediate receptor-dependent bacterial interactions, or coaggregation, in A. oris. In the first part of this dissertation, I identify components of the electron transport chain (ETC) required for pilus assembly, by characterizing A. oris Tn5 transposon mutants defective in coaggregation. Analyses of non-polar deletion mutants of nuo genes, encoding the NADH-dehydrogenase subunits, and ubiE, a menaquinone C-methyltransferase encoding-gene, confirmed defects in reactivation of MdbA. Our findings indicate these ETC components are biochemically linked to pilus assembly via oxidative protein folding. Because deletion of mdbA causes a temperature-sensitive growth and cell division defect in C. diphtheriae, it was postulated that additional oxidoreductase enzymes compensate for the loss of mdbA at the permissive temperature. The second part of this dissertation focuses on the characterization of an alternate oxidoreductase denominated TsdA. I found that DmdbA compensatory mutants overexpressing TsdA harbor a mutation that creates a sigma factor sA extended promoter thereby resulting in increased promoter strength. I determined that expression of this oxidoreductase is induced at 40°C, suggesting a novel role for an oxidoreductase in resistance to heat stress. Last, I investigated the requirement of MdbA for oxidative folding of cell division factors in C. diphtheriae. Penicillin binding proteins (PBPs) synthesize the bacterial cell wall and are key components of the cell division machinery. I demonstrated that overexpression of corynebacterial PBPs predicted to have disulfide bonds significantly rescues the morphology defects of the ΔmdbA strain. Furthermore, MdbA was found to be required for PBP stability and function. Overall this dissertation provides insights into novel aspects of the reactivation, regulation and requirement for growth of the oxidative protein folding pathways in the actinobacterial pathogens A. oris and C. diphtheriae