Bacterial addiction module toxin Doc inhibits translation elongation through its association with the 30S ribosomal subunit

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Abstract
Bacterial toxin–antitoxin (TA) systems (or “addiction modules”) typically facilitate cell survival during intervals of stress by inducing a state of reversible growth arrest. However, upon prolonged stress, TA toxin action leads to cell death. TA systems have also been implicated in several clinically important phenomena: biofilm formation, bacterial persistence during antibiotic treatment, and bacterial pathogenesis. TA systems harbored by pathogens also serve as attractive antibiotic targets. To date, the mechanism of action of the majority of known TA toxins has not yet been elucidated. We determined the mode of action of the Doc toxin of the Phd-Doc TA system. Doc expression resulted in rapid cell growth arrest and marked inhibition of translation without significant perturbation of transcription or replication. However, Doc did not cleave mRNA as do other addiction-module toxins whose activities result in translation inhibition. Instead, Doc induction mimicked the effects of treatment with the aminoglycoside antibiotic hygromycin B (HygB): Both Doc and HygB interacted with 30S ribosomal subunits, stabilized polysomes, and resulted in a significant increase in mRNA half-life. HygB also competed with ribosome-bound Doc, whereas HygB-resistant mutants suppressed Doc toxicity, suggesting that the Doc-binding site includes that of HygB (i.e., helix 44 region of 16S rRNA containing the A, P, and E sites). Overall, our results illuminate an intracellular target and mechanism of TA toxin action drawn from aminoglycoside antibiotics: Doc toxicity is the result of inhibition of translation elongation, possibly at the translocation step, through its interaction with the 30S ribosomal subunit.