Siderophores are low molecular weight, avid ferric ion-binding compounds synthesized by fungi and bacteria. They sequester ferric ion from environments where its concentration is critically low. Without siderophores, microbes in such environments would cease growth due to lack of iron. Siderophores are employed by microbes to supply iron in environments such as soil, water and clinical infections.

We are examining the fate of siderophores in the environment, specifically, how are siderophores degraded and return to the natural carbon and nitrogen cycles. We have isolated a soil bacterium (a Mesorhizobium loti) that utilizes the siderophore deferrioxamine B (DFB) as its sole source of carbon and is thus able to return the molecule to the carbon cycle. Work has begun to elucidate the mechanism and the enzymes by which the bacterium is able to use DFB as a carbon source. The catalyst responsible for the initial breakdown of DFB, which we call DFB hydrolase, may be a single enzyme or an enzyme consortium. Biochemical investigations revealed that the enzyme begins the dismantling of DFB by generating its constituent units, termed monohydroxamates. Molecular biology studies have resulted in "knocking out" DFB metabolism in M. loti mutants and tagging the responsible genes with a transposon (Tn5:OT182). We are in the process of cloning and sequencing these tagged genes in order to better understand what genes may be responsible for the degradation of DFB by M. loti.

Our personnel are also investigating the metabolism of nitrogen by cave bacteria. Our colleague, Dr. Hazel Barton of the University of Northern Kentucky, has provided isolates of pseudomonads (a type of bacterium) from a cave in Kentucky. We have examined these isolates for their ability to denitrify (the process of reducing nitrate or nitrite to nitrous oxide or nitrogen gas) and determined that only 5 of the 24 isolates were capable of denitrification even though the key gene of denitrificaiton (nirS or nirK) was present in 6 of the isolates. Work is ongoing to determine if other nitrogen metabolism genes, such as those for ammonification, are present in the isolates.