Non-canonical activation of OmpR drives acid and osmotic stress responses in single bacterial cells

Abstract

Unlike eukaryotes, bacteria undergo large changes in osmolality and cytoplasmic pH. It has been described that during acid stress, bacteria internal pH promptly acidifies, followed by recovery. Here, using pH imaging in single living cells, we show that following acid stress, bacteria maintain an acidic cytoplasm and the osmotic stress transcription factor OmpR is required for acidification. The activation of this response is non-canonical, involving a regulatory mechanism requiring the OmpR cognate kinase EnvZ, but not OmpR phosphorylation. Single cell analysis further identifies an intracellular pH threshold ~6.5. Acid stress reduces the internal pH below this threshold, increasing OmpR dimerization and DNA binding. During osmotic stress, the internal pH is above the threshold, triggering distinct OmpR-related pathways. Preventing intracellular acidification of Salmonella renders it avirulent, suggesting that acid stress pathways represent a potential therapeutic target. These results further emphasize the advantages of single cell analysis over studies of population averages.

The ArcAB two-component regulatory system promotes resistance to reactive oxygen species and systemic infection by Salmonella Typhimurium

“…This has been previously observed by others under different experimental conditions [71–75], and also noticed in our global transcriptomic analyses in the presence of H 2 O 2 and HOCl (in preparation). It is possible that ArcAB may be noncanonically activated under certain metabolic conditions.…”

To ∼P or Not to ∼P? Non‐canonical activation by two‐component response regulators

“…OmpR binding and repression does not require phosphorylation (i.e., a D55A mutant is sufficient to repress the cadC/BA operon), although it does require the cognate HK EnvZ or the cytoplasmic domain of EnvZ, EnvZc. The assumption is that interaction of EnvZ with OmpR is sufficient to drive OmpR dimerization and DNA binding in the absence of phosphorylation (Chakraborty et al ., ). A similar mechanism appears to function with the closely related OmpR homologue CpxR, where studies in Legionella pneumophila showed that a D53A CpxR mutant was no longer capable of transcriptional activation, but was still competent for repression (Feldheim et al ., ).…”

Engineering an Osmosensor by Pivotal Histidine Positioning within Disordered Helices

“…A particularly well-characterized TCS system is the EnvZ/OmpR system that is responsible for sensing changes in intracellular osmotic and acid environments, resulting in changes in the transcription profile (Chakraborty et al, 2015(Chakraborty et al, , 2017Wang et al, 2012). EnvZ is autophosphorylated (Aiba et al, 1989;Igo and Silhavy, 1988) on a conserved histidine residue, His 243 (Forst et al, 1987;Forst and Roberts, 1994), and osmolality enhances EnvZ autophosphorylation .…”