Spx inhibits expression of the SwrA*DegU master flagellar activator in Bacillus subtilis.
Flagella are trans-envelope nanomachines expressed from genes organized in a complex regulatory hierarchy governed at the highest level by transcription factors called master activators. The master activator of flagellar biosynthesis in Bacillus subtilis is a hybrid of SwrA*DegU that is required to increase flagellar density to swarm over solid surfaces. Here we show that the ClpX unfoldase subunit of the ClpP protease is required for swarming motility, and that cells mutated for ClpX fail to swarm due to restricted levels of both SwrA and DegU. Suppressor mutations were found that increased expression of the fla/che operon under SwrA*DegU control, and mutation of the LonA protease elevates the levels of SwrA protein while mutation of the global transcriptional regulator Spx increases transcription of both the degU and swrA genes. We conclude that ClpX promotes swarming motility via degradation of Spx, which represses motility gene transcription including the Pfla/che, PdegU, and PswrA promoters each activated by DegU. The ClpX-dependent regulatory proteolysis of Spx is relieved upon stress conditions, and we infer that Spx may dampen DegU-mediated positive feedback to limit cell envelope stress caused by excessive flagellar biosynthesis.