Spx inhibits expression of the SwrA•DegU master flagellar activator in <i>Bacillus subtilis</i>

Bacterial flagella are elaborate machines that are inserted through the bacterial cell envelope. The regulation of flagellar gene expression is organized in hierarchical tiers that largely correlate with the order of flagellar assembly. Here, we show that the abundance of the master activator of flagellar gene expression in <i>Bacillus subtilis</i> is restricted when the transcription factor Spx accumulates. Spx is an unusual transcription factor, as it binds to RNA polymerase and renders it insensitive to transcriptional activators. We show that Spx interferes with flagellar promoters activated by the response regulator DegU and may do so to limit a DegU-mediated positive feedback loop.