The two-component system ChvGI maintains cell envelope homeostasis in Caulobacter crescentus

Abstract

Two-component signal transduction systems (TCS) are often used by bacteria to rapidly assess and respond to environmental changes. ChvG/ChvI is a TCS conserved in α-proteobacteria and known for regulating expression of genes related to exopolysaccharide production, virulence and growth. The sensor kinase ChvG autophosphorylates upon yet unknown signals and phosphorylates the response regulator ChvI to activate transcription. Recent studies in Caulobacter crescentus showed that chv mutants are sensitive to vancomycin treatment and fail to grow in synthetic minimal media. In this work, we identified the osmotic imbalance as the main cause of growth impairment in synthetic minimal media. We also determined the ChvI regulon and confirmed that ChvI regulates cell envelope architecture at different levels by controlling outer membrane, peptidoglycan assembly/recycling and inner membrane proteins. Furthermore, we identified genes with osmoregulatory properties and confirmed that osmotic upshift is a signal triggering ChvG-dependent phosphorylation of ChvI. In addition, we challenged chv mutants with other cell envelope related stress and found that targeting with antibiotics the transpeptidation of peptidoglycan during cell elongation impairs growth of the mutant. Moreover, these antibiotics activate expression of the chvIG-hprK operon in ChvI-dependent and independent ways. Finally, we observed that the sensor kinase ChvG fused to a fluorescent protein relocates from a patchy-spotty distribution to distinctive foci after transition from complex to synthetic minimal media. Interestingly, this pattern of (re)location has been described for proteins involved in cell growth control and peptidoglycan synthesis upon osmotic shock. Overall, our data support that the ChvGI TCS is mainly used to maintain cell envelope homeostasis by monitoring osmotic imbalances and damages in the peptidoglycan layer.

Activation of ChvG-ChvI regulon by cell wall stress confers resistance to β-lactam antibiotics and initiates surface spreading inAgrobacterium tumefaciens

“…However, previous studies found no common response to stresses including osmotic stress, heat shock, and low pH in A. tumefaciens [43]. Overall, the data presented here are in agreement with recent works in Caulobacter [35,42] which suggest that ChvGI activation is important in oligotrophic free-living bacteria as a cell envelope or osmotic stress response.…”

“…The increased sensitivity to mecillinam, vancomycin, cefsulodin, and moenomycin in ChvGI mutants suggests that this pathway may confer resistance to antibiotics inhibiting cell wall synthesis [35]. Our observation that the depletion of A.…”

“…tumefaciens. Cells were spotted on agarose pads containing calcofluor white and grown overnight, then imaged using the DAPI filter to detect succinoglycan production (Fig Caulobacter crescentus where it was shown that ChvGI activation was required for osmoregulation [35]. Together, these findings indicate a conserved role within the Alphaproteobacteria for the ChvG-ChvI TCS in sensing and responding to envelope stress.…”

“…The overproduction of succinoglycan provides cells with passive protection against several stresses A. tumefaciens may encounter during host invasion including detergents, salt, acidity, heat, antimicrobial peptides, and reactive oxygen species [33][34][35][36]. Production of succinoglycan may also help protect against cell-wall-synthesis targeting antibiotics produced by competing bacteria and fungi in the soil.…”

“…Recent interest in the ChvG-ChvI pathway of Caulobacter crescentus (ChvGI) provides a glimpse at the function of the pathway in the context of a bacterium with a drastically different ecological niche to that of A. tumefaciens or S. meliloti . ChvGI of C. crescentus senses and responds to osmotic stress and mutants of ChvGI are sensitive to several cell-wall targeting antibiotics [12,13]. It remains unclear if this function is solely a characteristic of C. crescentus ChvGI or if it is conserved across ChvG-ChvI orthologs.…”