Bacterial siderophores, organic molecules that bind and transport Fe(III), are important in microbial interactions. They contribute to intermicrobial competition in the environment, as well as contributing to virulence. Shipworms are bivalve mollusks related to clams that burrow into and eat wood. They contain intracellular proteobacterial symbionts in their gills that assist them in wood degradation by producing cellulases that are transported to the wood digestion organ (cecum). The wood digestion organ is surprisingly depauperate of bacteria considering it is the site of breakdown of cellulose to sugars and that it is exposed to the external mileu. Iron restriction is a common mechanism of host suppression of unwanted microbes, and we hypothesized that shipworm gill symbionts might produce siderophores that could contribute to opportunist microbe suppression, or mediate interactions among the gill symbiont community. With the help of Alison Butler’s group, we identified a novel family of ornithine-containing catecholate siderophores in the shipworm symbiont Teredinibacter turnerae T7901, dubbed the turnerbactins, (similar to vanchrobactins and chrysobactins), and demonstrated that they are present in the host system. We identified the biosynthetic pathway, containing biosynthetic, transport and other genes. We developed a genetic system that allowed us to make targeted in-frame deletions of specific genes, and complement them. We verified the link between the biosynthetic pathway and turnerbactin production. We created a sensitive bioassay strain deleted in biosynthesis genes that allows us to screen our collection of shipworm symbionts for turnerbactin production. We showed that expression is regulated by iron, and using a Fur titration assay in E. coli, that Fur is a key regulator. We identified the outer membrane receptor for turnerbactin, FttA, and showed it is indispensable for turnerbactin uptake. Iron acquisition and competition is likely as important in this symbiotic system as in other bacterial habitats.

Siderophores in a symbiotic system

255th National Meeting (2018)

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