The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

Teredinibacter turnerae is an intracellular bacterial symbiont that resides in the gills of shipworms, wood-eating bivalve mollusks. This bacterium produces a catechol siderophore, turnerbactin, required for the survival of this bacterium under iron limiting conditions. The turnerbactin biosynthetic genes are contained in one of the secondary metabolite clusters conserved among T. turnerae strains. However, Fe(III)-turnerbactin uptake mechanisms are largely unknown. Here, we show that the first gene of the cluster, fttA a homologue of Fe(III)-siderophore TonB-dependent outer membrane receptor (TBDR) genes is indispensable for iron uptake via the endogenous siderophore, turnerbactin, as well as by an exogenous siderophore, amphi-enterobactin, ubiquitously produced by marine vibrios. Furthermore, three TonB clusters containing four tonB genes were identified, and two of these genes, tonB1b and tonB2, functioned not only for iron transport but also for carbohydrate utilization when cellulose was a sole carbon source. Gene expression analysis revealed that none of the tonB genes and other genes in those clusters were clearly regulated by iron concentration while turnerbactin biosynthesis and uptake genes were up-regulated under iron limiting conditions, highlighting the importance of tonB genes even in iron rich conditions, possibly for utilization of carbohydrates derived from cellulose.