Utilization of 5-deoxy-nucleosides as Growth Substrates by Extraintestinal Pathogenic E. coli via the Dihydroxyacetone Phosphate Shunt

Abstract

All organisms utilize S-adenosyl-L-methionine (SAM) as a key co-substrate for methylation of biological molecules, synthesis of polyamines, and radical SAM reactions. When these processes occur, 5-deoxy-nucleosides are formed as byproducts such as S-adenosyl-L-homocysteine (SAH), 5-methylthioadenosine (MTA), and 5-deoxyadenosine (5dAdo). One of the most prevalent pathways found in bacteria for the metabolism of MTA and 5dAdo is the DHAP shunt, which converts these compounds into dihydroxyacetone phosphate (DHAP) and 2-methylthioacetaldehyde or acetaldehyde, respectively. Previous work has shown that the DHAP shunt can enable methionine synthesis from MTA or serve as an MTA and 5dAdo detoxification pathway. Here we show that in Extraintestinal Pathogenic E. coil (ExPEC), the DHAP shunt serves none of these roles in any significant capacity, but rather physiologically functions as an assimilation pathway for use of MTA and 5dAdo as growth substrates. This is further supported by the observation that when MTA is the substrate for the ExPEC DHAP shunt, the sulfur components is not significantly recycled back to methionine, but rather accumulates as 2-methylthioethanol, which is slowly oxidized non-enzymatically under aerobic conditions. While the pathway is active both aerobically and anaerobically, it only supports aerobic ExPEC growth, suggesting that it primarily functions in oxygenic extraintestinal environments like blood and urine versus the predominantly anoxic gut. This reveals a heretofore overlooked role of the DHAP shunt in carbon assimilation and energy metabolism from ubiquitous SAM utilization byproducts and suggests a similar role may occur in other pathogenic and non-pathogenic bacteria with the DHAP shunt. ImportanceAcquisition and utilization of organic compounds that can serve as growth substrates is essential for pathogenic E. coli to survive and multiply. Ubiquitous enzymatic reactions involving S-adenosyl-L-methionine as a co-substrate result in the formation of the 5-deoxy-nucleoside byproducts, 5-methylthioadenosine and 5-deoxyadenosine. All E. coli possess a conserved nucleosidase that cleaves these 5-deoxy-nucleosides into 5-deoxy-pentose sugars for adenine salvage. The DHAP shunt pathway, which is found in ExPEC strains but neither in intestinal pathogenic nor commensal E. coli, enables utilization of 5-deoxy-nucleosides and 5-deoxy-pentose sugars as growth substrates by ExPEC strains. This provides insight into the diversity of sugar compounds accessible by ExPEC strains in recalcitrant and nutrient-poor environments such as the urinary tract during infection. Furthermore, given the dihydroxyacetone phosphate shunt pathway appears to only support aerobic E. coli growth, this suggests an explanation as to why intestinal strains that primarily exist in anoxic environments lack this pathway.