A regulatory pathway that selectively up-regulates elongasome function in the absence of class A PBPs.

Abstract

Bacteria surround themselves with peptidoglycan, an adaptable enclosure that contributes to cell shape and stability. Peptidoglycan assembly relies on penicillin-binding proteins (PBPs) acting in concert with SEDS-family transglycosylases RodA and FtsW, which support cell elongation and division respectively. In <i>Bacillus subtilis</i>, cells lacking all four PBPs with transglycosylase activity (aPBPs) are viable. Here, we show that the alternative sigma factor σ^I is essential in the absence of aPBPs. Defects in aPBP-dependent wall synthesis are compensated by σ^I-dependent upregulation of an MreB homolog, MreBH, which localizes the LytE autolysin to the RodA-containing elongasome complex. Suppressor analysis reveals that cells unable to activate this σ^I stress response acquire gain-of-function mutations in the essential histidine kinase WalK, which also elevates expression of <i>sigI</i>, <i>mreBH</i> and <i>lytE</i>. These results reveal compensatory mechanisms that balance the directional peptidoglycan synthesis arising from the elongasome complex with the more diffusive action of aPBPs.