Biosynthesis of diverse siderophores in Shewanella oneidensis: a result of promiscuous enzymes

The siderophore synthetic system in Shewanella species is able to synthesize dozens of macrocyclic siderophores in vitro with synthetic precursors. In vivo, however, although three siderophores are produced naturally in S. algae B516 which carries a lysine decarboxylase (AvbA) specific for siderophore synthesis, only one siderophore can be detected from many other Shewanella species. In this study, we examined a siderophore-overproducing mutant of S. oneidensis, which lacks an AvbA counterpart, and found that it can also produce these three siderophores. We identified both SpeC and SpeF as promiscuous decarboxylases for both lysine and ornithine to synthesize siderophore precursors cadaverine and putrescine respectively. Intriguingly, putrescine is mainly synthesized from arginine through an arginine decarboxylation pathway in a constitutive manner, not liable to the concentrations of iron and siderophores. Our results provide further evidence that the substrate availability plays a determining role in siderophore production. Furthermore, we provide evidence to suggest that under iron starvation conditions, cells allocate more putrescine for siderophore biosynthesis by down-regulating expression of the enzyme that transforms putrescine into spermidine. Overall, this study provides another example of the great flexibility of bacterial metabolism that is honed by evolution to better fit living environments of these bacteria. Importance The simultaneous production of multiple siderophores is considered a general strategy for microorganisms to rapid adapt to their ever-changing environments. In this study, we show that some Shewanella may downscale their capability of siderophore synthesis to facilitate adaptation. Although S. oneidensis lacks an enzyme specifically synthesizing cadaverine, it can produce it by using promiscuous ornithine decarboxylases. Despite the ability, this bacterium predominately produces the primary siderophore while restraining production of secondary siderophores by regulating substrate availability. In addition to use the ADC pathway for putrescine synthesis, cells optimize the putrescine pool for siderophore production. Our work provides an insight into coordinated synthesis of multiple siderophores by harnessing promiscuous enzymes in bacteria and underscores the importance of substrate pools for biosynthesis of natural products.