The alternative sigma factor SigN of Bacillus subtilis is intrinsically toxic

Sigma factors bind and direct the RNA polymerase core to specific promoter sequences and alternative sigma factors direct transcription of different regulons of genes. Here, we study the pBS32 plasmid-encoded sigma factor SigN of Bacillus subtilis to determine how it contributes to DNA damage-induced cell death. We find that SigN causes cell death when expressed at high level and does so in the absence of its regulon suggesting it is intrinsically toxic. One way toxicity was relieved was by curing the pBS32 plasmid, which eliminated a positive feedback loop that lead to SigN hyper-accumulation. Another way toxicity was relieved was through mutating the chromosomally-encoded transcriptional repressor protein AbrB and derepressing a potent antisense transcript that antagonized SigN expression. We note that SigN exhibits a relatively high affinity for the RNA polymerase core, efficiently competing with the vegetative sigma factor SigA, suggesting that toxicity was due to the competitive inhibition of one or more essential transcripts. Why B. subtilis encodes a potentially toxic sigma factor is unclear but SigN may be related to phage-like genes also encoded on pBS32.

Alternative sigma factors activate entire regulons of genes to improve viability in response to environmental stimuli. The pBS32 plasmid-encoded SigN of Bacillus subtilis is activated by the DNA damage response and leads to cellular demise. Here we find that SigN impairs viability by hyper-accumulating and outcompeting the vegetative sigma factor for the RNA polymerase core. Why B. subtilis retains a plasmid with a deleterious alternative sigma factor is unknown.