The Bacillus subtilis iron-sparing response is mediated by a Fur-regulated small RNA and three small, basic proteins.

Regulation of bacterial iron homeostasis is often controlled by the iron-sensing ferric uptake repressor (Fur). The Bacillus subtilis Fur protein acts as an iron-dependent repressor for siderophore biosynthesis and iron transport proteins. Here, we demonstrate that Fur also coordinates an iron-sparing response that acts to repress the expression of iron-rich proteins when iron is limiting. When Fur is inactive, numerous iron-containing proteins are down-regulated, including succinate dehydrogenase, aconitase, cytochromes, and biosynthetic enzymes for heme, cysteine, and branched chain amino acids. As a result, a fur mutant grows slowly in a variety of nutrient conditions. Depending on the growth medium, rapid growth can be restored by mutations in one or more of the molecular effectors of the iron-sparing response. These effectors include the products of three Fur-regulated operons that encode a small RNA (FsrA) and three small, basic proteins (FbpA, FbpB, and FbpC). Extensive complementarity between FsrA and the leader region of the succinate dehydrogenase operon is consistent with an RNA-mediated translational repression mechanism for this target. Thus, iron deprivation in B. subtilis activates pathways to remodel the proteome to preserve iron for the most critical cellular functions.