The stressosome is required to transduce low pH signals leading to increased transcription of the amino acid-based acid tolerance mechanisms in Listeria monocytogenes

Abstract

Increasing proton concentration in the environment represents a potentially lethal stress for single-celled microorganisms. To survive in an acidifying environment, the foodborne pathogen Listeria monocytogenes quickly activates the alternative sigma factor B (σ B ), resulting in upregulation of the general stress response (GSR) regulon. Activation of σ B is regulated by the stressosome, a multi-protein sensory complex involved in stress detection and signal transduction. In this study, we used L. monocytogenes strains harbouring two stressosome mutants to investigate the role of this complex in triggering expression of known amino acid-based resistance mechanisms in response to low pH. We found that expression of glutamate decarboxylase (gadD3) and arginine and agmatine deiminases (arcA and aguA1, respectively) were upregulated upon acid shock (pH 5 for 15 min) in a stressosome-dependent manner. In contrast, transcription of the arg operons (argGH and argCJBDF), which encode enzymes for the l-arginine biosynthesis pathway, were upregulated upon acid shock in a stressosome-independent manner. Finally, we found that transcription of argR, which encodes a transcriptional regulator of the arc and arg operons, was largely unaffected by acidic shock. Thus, our findings suggest that the stressosome plays a role in activating amino acid-based pH homeostatic mechanisms in L. monocytogenes. Additionally, we show that genes encoding the l-arginine biosynthesis pathway are highly upregulated under acidic conditions, suggesting that intracellular arginine can help withstand environmental acidification in this pathogen.

A novel RofA-family transcriptional regulator, GadR, controls the development of acid resistance inListeria monocytogenes

“…The total GAD activity is possibly supported by SigB-mediated gadD3 expression, while other proton-consuming mechanisms (e.g., arginine/agmatine deaminase) might also contribute to the overall acid resistance [20]. SigB as the general stress response regulator promotes an array of mechanisms that contribute to intracellular pH maintenance (S2 Table) [19,30]. Our data suggest that there is an additive (rather than synergistic) effect in acid resistance between GadR and SigB.…”

“…Little is known about the transcriptional regulation of these two systems, although the transcription of gadD3 is under SigB control [30,31]. SigB also controls the expression of succinate semialdehyde dehydrogenase (Lmo0913), which is required in GABA-shunt pathway that generates succinate and Glu from GABA and α-ketoglutarate [15,32].…”

Signals behind Listeria monocytogenes virulence mechanisms

“… 53-56 Moreover, the activation of σ β by the stressosome is also required for the activation of the glutamate decarboxylase enzyme gadD3 , that converts L-glutamate into γ-aminobutyrate (GABA) while consuming an intracellular proton, and of the arginine ( arcA ) and agmatine ( aguA1 ) deiminases, which metabolize L-arginine into citrulline and ammonia to adjust the cytoplasmic pH. 54 , 181 During the first process, the consumption of one proton itself is enough to increase the intracellular pH thus attenuating acidic stress. 182 GadD3 together with GadD1 and GadD2 enzymes and the glutamate/GABA antiporters (GadT1 and T2) form the glutamate decarboxylase (GAD) system which is key to maintain Lm pH homeostasis in acidic environments.…”