Septal Localization of the Mycobacterium tuberculosis MtrB Sensor Kinase Promotes MtrA Regulon Expression

Abstract

The mechanisms responsible for activation of the MtrAB two-component regulatory signal transduction system, which includes sensor kinase MtrB and response regulator MtrA, are unknown. Here, we show that an MtrB-GFP fusion protein localized to the cell membrane, the septa, and the poles in Mycobacterium tuberculosis and Mycobacterium smegmatis . This localization was independent of MtrB phosphorylation status but dependent upon the assembly of FtsZ, the initiator of cell division. The M. smegmatis mtr B mutant was filamentous, defective for cell division, and contained lysozyme-sensitive cell walls. The mtr B phenotype was complemented by either production of MtrB protein competent for phosphorylation or overproduction of MtrA Y102C and MtrA D13A mutant proteins exhibiting altered phosphorylation potential, indicating that either MtrB phosphorylation or MtrB independent expression of MtrA regulon genes, including those involved in cell wall processing, are necessary for regulated cell division. In partial support of this observation, we found that the essential cell wall hydrolase rip A is an MtrA target and that the expression of bona fide MtrA targets rip A, fbp B, and dna A were compromised in the mtr B mutant and partially rescued upon MtrA Y102C and MtrA D13A overproduction. MtrB septal assembly was compromised upon FtsZ depletion and exposure of cells to mitomycin C, a DNA damaging agent, which interferes with FtsZ ring assembly. Expression of MtrA targets was also compromised under the above conditions, indicating that MtrB septal localization and MtrA regulon expression are linked. We propose that MtrB septal association is a necessary feature of MtrB activation that promotes MtrA phosphorylation and MtrA regulon expression. Background: The MtrB sensor kinase is a component of the essential MtrAB signal transduction system. Results: MtrB localizes to septa independent of phosphorylation; MtrB septal association and phosphorylation are necessary for cell division and expression of MtrA targets. Conclusion: MtrB septal localization and MtrA-regulon expression are linked. Significance: MtrB septal assembly triggers the activation of the MtrAB signal transduction pathway.