Role of Lon and ClpX in the post-translational regulation of a sigma subunit of RNA polymerase required for cellular differentiation in Bacillus subtilis

The RNA polymerase sigma subunit, sigma(H) (Spo0H) of Bacillus subtilis, is essential for the transcription of genes that function in sporulation and genetic competence. Although spo0H is transcriptionally regulated by the key regulatory device that controls sporulation initiation, the Spo0 phosphorelay, there is considerable evidence implicating a mechanism of post-translational control that governs the activity and concentration of sigma(H). Post-translational control of spo0H is responsible for the reduced expression of genes requiring sigma(H) under conditions of low environmental pH. It is also responsible for heightened sigma(H) activity upon relief of acid stress and during nutritional depletion. In this study, the ATP-dependent proteases LonA and B and the regulatory ATPase ClpX were found to function in the post-translational control of sigma(H). Mutations in lonA and lonB result in elevated sigma(H) protein concentrations in low-pH cultures. However, this is not sufficient to increase sigma(H)-dependent transcription. Activation of sigma(H)-dependent transcription upon raising medium pH and in cells undergoing sporulation requires clpX, as shown by measuring the expression of lacZ fusions that require sigma(H) for transcription and by complementation of a clpX null mutation. A hypothesis is presented that low environmental pH results in the Lon-dependent degradation of sigma(H), but the activity of sigma(H) in sporulating cells and in cultures at neutral pH is stimulated by a ClpX-dependent mechanism in response to nutritional stress.