Role of RsbU in Controlling SigB Activity in Staphylococcus aureus following Alkaline Stress

SigB is an alternative sigma factor that controls a large regulon inStaphylococcus aureus . Activation of SigB requires RsbU, a protein phosphatase 2C (PP2C)-type phosphatase. In a closely related organism,Bacillus subtilis , RsbU activity is stimulated upon interaction with RsbT, a kinase, which following an activating stimulus switches from a 25S high-molecular-weight complex, the stressosome, to the N-terminal domain of RsbU. Active RsbU dephosporylates RsbV and thereby triggers the release of SigB from its inhibitory complex with RsbW. While RsbU, RsbV, RsbW, and SigB are conserved inS. aureus , proteins similar to RsbT and the components of the stressosome are not, raising the question of how RsbU activity and hence SigB activity are controlled inS. aureus . We found that in contrast to the case inB. subtilis , the induced expression of RsbU was sufficient to stimulate SigB-dependent transcription inS. aureus . However, activation of SigB-dependent transcription following alkaline stress did not lead to a clear accumulation of SigB and its regulators RsbV and RsbW or to a change in the RsbV/RsbV-P ratio inS. aureus . When expressed inB. subtilis , theS. aureus RsbU displayed a high activity even in the absence of an inducing stimulus. This high activity could be transferred to the PP2C domain of theB. subtilis RsbU protein by a fusion to the N-terminal domain of theS. aureus RsbU. Collectively, the data suggest that the activity of theS. aureus RsbU and hence SigB may be subjected to different regulation in comparison to that inB. subtilis .