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, σ(H) (SpoOH) of Bacillus subtilis, is essential for the transcription of genes that function in sporulation and genetic competence. Although spoOH is transcriptionally regulated by the key regulatory device that controls sporulation initiation, the SpoO phosphorelay, there is considerable evidence implicating a mechanism of post-translational control that governs the activity and concentration of σ(H). Post-translational control of spoOH is responsible for the reduced expression of genes requiring σ(H) under conditions of low environmental pH. It is also responsible for heightened σ(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 σ(H). Mutations in lonA and lonB result in elevated σ(H) protein concentrations in low-pH cultures. However, this is not sufficient to increase σ(H) dependent transcription. Activation of σ(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 σ(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 σ(H), but the activity of σ(H) in sporulating cells and in cultures at neutral pH is stimulated by a ClpX-dependent mechanism in response to nutritional stress.