The Major Facilitator Superfamily-Type Transporter YmfE and the Multidrug-Efflux Activator Mta Mediate Bacillibactin Secretion inBacillus subtilis

High-affinity iron acquisition inBacillus subtilis is mediated via the bacillibactin catechole siderophore pathway. Three of the four essential pathway steps, bacillibactin synthesis, Fe-bacillibactin uptake, and Fe-bacillibactin hydrolysis have been characterized previously. The functional and regulatory components for bacillibactin secretion, the second step of the siderophore pathway, remained unknown. In this study, the screening of aB. subtilis exporter mutant library led to the identification of the YmfE major facilitator superfamily (MFS)-type transporter as a target for bacillibactin export. Analysis of iron-limitedymfE mutant cultures displayed an eightfold reduced bacillibactin secretion and, on the other hand, a 25-fold increased secretion of the bacillibactin precursor 2,3-dihydroxybenzoate. Investigation of the regulatory aspect revealed that bacillibactin secretion is, in contrast to all other components of the pathway, independent of the ferric uptake repressor Fur. Indeed, the MerR-type transcriptional regulator Mta was found to activate both bacillibactin secretion andymfE gene expression, exposing Mta as an additional regulatory member of the bacillibactin pathway.