Natural combinatorial genetics and prolific polyamine production enable siderophore diversification in Serratia plymuthica

Siderophores are small molecules with unmatched capacity to scavenge iron from proteins and the extracellular milieu, where it mostly occurs as insoluble Fe 3+ . Siderophores chelate Fe 3+ for uptake into the cell, where it is reduced to soluble Fe 2+ . As iron is essential for bacterial survival, siderophores are key molecules in low soluble iron conditions. Bacteria have devised many strategies to synthesize proprietary siderophores to avoid siderophore piracy by competing organisms, e.g., by incorporating different polyamine backbones into siderophores, while maintaining the catechol moieties. We report that Serratia plymuthica V4 produces a variety of siderophores, which we term the siderome , and which are assembled by the concerted action of enzymes encoded in two independent gene clusters. Besides assembling serratiochelin with diaminopropane, S. plymuthica utilizes putrescine and the same set of enzymes to assemble photobactin, a siderophore described for Photorhabdus luminescens . The enzymes encoded by one of the gene clusters can independently assemble enterobactin. A third, independent operon is responsible for biosynthesis of the hydroxamate siderophore aerobactin, initially described in Enterobacter aerogenes . Mutant strains not synthesizing polyamine-siderophores significantly increased enterobactin production levels, though lack of enterobactin did not impact serratiochelin production. Knocking out SchF0, an enzyme involved in the assembly of enterobactin alone, significantly reduced bacterial fitness. This study illuminates the interplay between siderophore biosynthetic pathways and polyamine production superpathways, indicating routes of molecular diversification. Given its natural yields of diaminopropane (97.75 μmol/g DW) and putrescine (30.83 μmol/g DW), S. plymuthica can be exploited for the industrial production of these compounds. Significance Statement Siderophores are molecules crucial for bacterial survival in low iron environments. Bacteria have evolved the capacity to pirate siderophores made by other bacterial strains and to diversify the structure of their own siderophores, to prevent piracy. We found that Serratia plymuthica V4 produces five different siderophores using three gene clusters and a polyamine production superpathway. The most well studied siderophore, enterobactin, rather than the strain’s proprietary and by far most abundant siderophore, serratiochelin, displayed a crucial role in the fitness of S. plymuthica . Our results also indicate that this strain is a good candidate for engineering the large-scale production of diaminopropane (DAP), as without any optimization it produced the highest amounts of DAP reported for wild-type strains.