The iron chelator pulcherriminic acid mediates the light response in Bacillus subtilis biofilms

Non-photosynthetic bacteria often respond to changes in light. These responses are usually regulated by photoreceptor proteins, but the mechanism of light response in biofilms is poorly understood. Here, we show that colony biofilms of Bacillus subtilis display light responses that are not dependent on typical photoreceptor proteins. Under light, B. subtilis biofilms do not mature and instead keep on expanding, resulting in thin, smooth colonies with low pigmentation. Similar effects have been previously observed upon inhibition of the biosynthesis or export of pulcherriminic acid, an iron chelator known to inhibit colony expansion by reducing extracellular iron levels. We show that light induces spontaneous degradation of pulcherriminic acid. In addition, Fe³⁺-bound pulcherriminic acid upregulates the yvmC operon (which is responsible for pulcherriminic acid biosynthesis) by inhibiting the binding of a repressor protein (PchR) to the yvmC promoter. Thus, the photosensitivity of pulcherriminic acid enables this metabolite to control iron availability, yvmC expression, and biofilm development in response to changes in light conditions.