Salvage of the sulfur and carbon byproducts of S-adenosylmethionine metabolism in pathogenic bacteria

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Project Summary Biologically available sulfur is essential for the synthesis of methionine (Met) and its derivative, S-adenosyl-L- methionine (SAM). SAM is used for diverse metabolic purposes, serving primarily as a methyl donor for DNA and protein methylation, as a 5’-deoxyadenosyl radical donor for radical-SAM reactions, as an aminopropyl donor for polyamine synthesis and volved in the synthesis of acyl-homoserine lactone quorum sensing molecules in bacteria. As a consequence of this metabolism, a dead-end, sulfur-containing byproduct, 5’-methylthioadenosine (MTA) is formed. MTA is a product inhibitor of polyamine synthesis and MTA accumulation is thought to be toxic. Since the assimilation of inorganic sulfur is energetically costly and many organisms encounter sulfur-poor environments, maintaining or salvaging appropriate cellular organic sulfur pools is critical. Moreover, disruption or reduced functioning of methionine salvage pathways (MSPs) has many health-related consequences including influences on cancer cell growth and liver cirrhosis; intermediates of the pathway have also been shown to influence apoptopic processes, while analogs of these intermediates are promising therapeutic agents. Newly discovered MTA pathways from our laboratory, the DHAP-ethylene and methanethiol shunts, were recently described, the genes of which appear to be widespread and selectively found among several pathogenic species. Nonpathogenic species from these genera do not contain these genes. Thus, the hypothesis is that the shunt genes/enzymes hold some special significance to metabolism of these pathogenic species. Moreover, the same novel genes and enzymes were recently found to participate in radical SAM reactions to generate and metabolize 5’-deoxyadenosine (5dAdo), a structurally similar byproduct to MTA, which could potentially be recycled for carbon salvage. The long-term goal will thus be to determine the role and physiological significance of the DHAP/MTA/5dAdo pathways for sulfur and carbon salvage, and the potential of these pathways to influence the successful metabolism of extraintestinal pathogenic Escherichia coli (ExPEC), including uropathogenic (UPEC) strains which contain these genes on a specific pathogenesis island. A specific aim (Aim 1) will be to determine the precise role of these genes and encoded enzymes and resolve further metabolic steps in sulfur/carbon salvage via whole cell feeding experiments using radio-labeled (14C) and 13C MTA and 5dAdo metabolites in wild type and mutant strains. These in vivo studies will be supplemented by in vitro analyses with specific enzymes. The second aim (Aim 2) will involve resolving how these genes are genetically regulated, an important facet of sulfur/carbon salvage in these organisms. Resolution of the specific aims of this project have considerable health relevance as ExPEC/UPEC strains cause major health problems and infect millions of people. It is conceivable that the identification and resolution of a specific sulfur/carbon salvage pathway essential for pathogenesis/fitness will open the way to design specific targets to inhibit infections caused by these organisms.