A cell–cell communication system regulates protease production during sporulation in bacteria of the Bacillus cereus group

Summary In sporulating Bacillus, major processes like viru- lence gene expression and sporulation are regulated by communication systems involving signalling pep- tides and regulators of the RNPP family. We investi- gated the role of one such regulator, NprR, in bacteria of the Bacillus cereus group. We show that NprR is a transcriptional regulator whose activity depends on the NprX signalling peptide. In association with NprX, NprR activates the transcription of an extracellular protease gene (nprA) during the first stage of the sporulation process. The transcription start site of the nprA gene has been identified and the minimal region necessary for full activation has been characterized by promoter mutagenesis. We demonstrate that the NprX peptide is secreted, processed and then reim- ported within the bacterial cell. Once inside the cell, the mature form of NprX, presumably the SKPDIVG heptapeptide, directly binds to NprR allowing nprA transcription. Alignment of available NprR sequences from different species of the B. cereus group defines seven NprR clusters associated with seven NprX heptapeptide classes. This cell-cell communication system was found to be strain-specific with a pos- sible cross-talk between some pherotypes. The phylogenic relationship between NprR and NprX sug- gests a coevolution of the regulatory protein and its signalling peptide.