Siderophores are avid ferric ion-chelating molecules that sequester the metal for microbes. Microbes elicit siderophores in numerous and different environments, but the means by which these molecules reenter the carbon and nitrogen cycles is poorly understood. The metabolism of the trihydroxamic acid siderophore deferrioxamine B by a Mesorhizobium loti isolated from soil was investigated. Specifically, the pathway by which the compound is cleaved into its constituent monohydroxamates was examined. High-performance liquid chromatography and mass-spectroscopy analyses demonstrated that M. loti enzyme preparations degraded deferrioxamine B, yielding a mass-to-charge (m/z) 361 dihydroxamic acid intermediate and an m/z 219 monohydroxamate. The dihydroxamic acid was further degraded to yield a second molecule of the m/z 219 monohydroxamate as well as an m/z 161 monohydroxamate. These studies indicate that the dissimilation of deferrioxamine B by M. loti proceeds by a specific, achiral degradation and likely represents the reversal by which hydroxamate siderophores are thought to be synthesized.Elemental iron (Fe) is required to synthesize a number of key enzymes and biomolecules (1,8,14,(24)(25)(26)). Iron's biological availability, however, is restricted in those environments where oxygen gas is present and the prevailing pH is near neutrality or is alkaline. As the solution to the problem of acquiring sufficient amounts of Fe, numerous microbes employ siderophores, that is, avid, organic, microbial ferric ion chelators which sequester iron from environments where it is in short supply. Siderophores are thus essential to the nutrition of microbes existing in environments that would otherwise limit their growth (1,8,14,24,25). Such environments include fresh and marine waters, divergent soils, and living organisms (8,10,14,16,(27)(28)(29). In these ecosystems, micromolar concentrations of siderophores have been noted (16,27,28).Siderophores are virulence factors for both animal and plant pathogens (2, 3, 9-11, 18, 23). Indeed, the sequestration of iron by the host is an innate immune mechanism that may limit the course of pathogenic infections (8,14). Much research has been conducted to investigate the biosynthesis, iron chelation, iron assimilation, and genetics which allow microbes to acquire iron via siderophores. Far less attention, however, has been given to the study of how siderophores are mineralized and returned to the carbon and nitrogen cycles.Three siderophore-degrading microbes, a pseudomonad (33-35), Azospirillum irakense (36, 37), and Mesorhizobium loti (4,7,17,39), catabolize siderophores concomitant with their growth. Neilands and colleagues (33-35) observed that their microbe, named Pseudomonas FC1, degraded the hydroxamate siderophores ferrichrome, ferrichrome A, and coprogen, with ferrichrome A being the most facile to degrade. Pseudomonas FC1 siderophore degradation was due to inducible enzymes and could occur with either the deferrisiderophore or the ferrisiderophore. The enzymes required to degr...