Flagellar toxicity: flagellar synthesis is lytic for Bacillus subtilis in the absence of PBP1

Flagella are large transenvelope nanomachines but how they transit the peptidoglycan in Gram positive bacteria is poorly understood. A recent model suggested that flagellar basal bodies diffuse in the membrane and become captured at locations in the peptidoglycan with a pore diameter that could accommodate the axle-like flagellar rod. Mutation of penicillin binding protein 1 (PBP1/PonA), a cell wall repair protein thought to decrease peptidoglycan pore frequency and/or size, resulted in a severe growth defect and cell lysis in the ancestral strain of Bacillus subtilis that was dependent on flagellar synthesis. Genetic analysis indicated that toxicity was due to completion of the flagellar hook, which activated the flagellar sigma factor SigD. SigD, in turn, activated a suite of peptidoglycan hydrolases that caused cellular lysis when PBP1 was absent. In addition, mutations that resulted in high levels of the stress response factor Spx could lessen the toxicity, while PBPX, a putative teichoic acid D-alanylase, was required for autolysis. In sum our results indicate that flagellar synthesis, not normally associated with cell viability, causes cell wall stress and under some conditions, cell death. Moreover, our work indicates that cost of envelope integrity by flagellar synthesis may be underappreciated due to strain domestication, and suggests that specialized systems may compensate for the cost of assembly of transenvelope machines in general. SIGNIFICANCEBacteria assemble nanomachines through the cell envelope but how the machines transit the peptidoglycan is poorly understood. Here we find that assembly of trans-envelope flagella results in cell lysis of Bacillus subtilis when the peptidoglycan repair protein PBP1 is absent. Lysis was due to multiple peptidoglycan lyases expressed as a consequence of flagellar assembly, and lytic activity required another PBP homolog, PBPX. Our work indicates that flagella, not normally thought to impact cell viability, can be lethal at the level of cell envelope integrity.