Bacterial Persistence to Antibiotic Treatment Mediated by HipA Toxin Inhibition of Glutamyl-tRNA-Synthetase Activity

Persisters, which are the main cause for relapsing and chronic infections, are not mutants, but rather are dormant cells that can survive the antimicrobial treatments that kill the majority of their genetically identical siblings. Persister cells have entered a non- or extremely slow-growing physiological state, which makes them insensitive to the action of antimicrobial drugs. When such persisting microbial cells cannot be eliminated by the immune system, they become a reservoir from which recurrence of infection will develop.

In a recent study investigators at the Hebrew University of Jerusalem (Israel) searched for a molecular explanation for bacterial persistence by screening an expression library for putative targets of HipA, the first toxin linked to persistence. HipA belongs to a family of phosphatidylinositide and protein kinases and is capable of autophosphorylation. It causes inhibition of macromolecular synthesis, but the mechanism of action is unknown. Mutants of HipA lacking either predicted active-site residues or the site of autophosphorylation are defective in producing multidrug-tolerant cells.

The investigators reported in the December 17, 2013, online edition of the journal Nature Communications that GltX, the glutamyl-tRNA-synthetase, reversed the toxicity of HipA and prevented the development of persistent forms. GltX is a member of the enzyme family that catalyzes the attachment of an amino acid to its cognate transfer RNA molecule in a highly specific two-step reaction.

Upon HipA expression, GltX activity was blocked, as the enzyme was phosphorylated at Ser239 (amino acid serine residue 239), its ATP-binding site. This phosphorylation led to accumulation of uncharged tRNA(Glu) in the cell, which resulted in the activation of the stringent response. The stringent response is a stress response of bacteria and plant chloroplasts in reaction to amino-acid starvation, fatty acid limitation, iron limitation, heat shock, and other stress conditions. The stringent response modulates transcription of up to one-third of all genes in the cell. This in turn causes the cell to divert resources away from growth and division and toward amino acid synthesis in order to promote survival until nutrient conditions improve.

The investigators stated that, "Our findings demonstrate a mechanism for persister formation by the hipBA toxin-antitoxin module and provide an explanation for the long-observed connection between persistence and the stringent response."

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Hebrew University of Jerusalem