The FsrA-Mediated Iron-Sparing Response Regulates the Biosynthesis of the Epipeptide EPE in Bacillus subtilis

Under severe nutrient-limiting conditions, Bacillus subtilis is able to form highly resilient endospores for survival. However, to avoid this irreversible process, it employs an adaptive strategy termed cannibalism, a form of programmed cell death, to outcompete siblings and delay sporulation. One of the three cannibalism toxins, the epipeptide EPE, is encoded by the epeXEPAB operon. The pre-pro-peptide EpeX undergoes post-translational modification and processing to be secreted as the mature EPE toxin. While EPE production is tightly regulated at multiple levels, this study focuses on the post-transcriptional control by the small regulatory RNA FsrA, which is transcriptionally regulated by the global iron response regulator Fur. Electrophoretic mobility shift assays and RNA structure probing revealed two binding sites of FsrA within the intergenic region between epeX and epeE flanking the annotated epeX terminator structure and potentially interfering with RNA stability and epeXEP expression. Reporter assays revealed decreased levels of EPE-dependent stress response in the absence of FsrA, indicative of a positive FsrA effect on gene expression under iron-limited conditions; in contrast to the normally inhibitory activity of FsrA. Together, our findings suggest that under iron starvation, FsrA promotes RNA processing and enables epeE translation, ultimately enhancing EPE production.