Antitoxin flips a bacterial toxin off by triggering self-phosphorylation

Bacteria use a variety of enzymes called nucleotidyltransferases (NTases) to manage essential tasks such as DNA replication and repair, antibiotic resistance, and RNA modification. One member of this enzyme family, the toxin AbiEii from Streptococcus agalactiae, was known to block bacterial growth, but researchers did not understand how it worked or how the cell keeps it in check. Pierre Genevaux and colleagues investigated this toxin and its partner protein, the antitoxin AbiEi, which had been reported to antagonize AbiEii as part of a Type IV toxin-antitoxin system. Using a combination of structural, biochemical, and biophysical approaches, the team set out to reveal how the two proteins interact and what happens to AbiEii when AbiEi is present. Their work shows that AbiEi does more than simply block access to a target; it binds AbiEii and triggers a chemical change in the toxin. This interaction ultimately stops the NTase activity of AbiEii and helps to safeguard global translation, clarifying how cells neutralize a growth-blocking enzyme and maintain core processes.

To uncover the mechanism, the researchers combined structural, biochemical, and biophysical studies to observe the AbiEi–AbiEii pair directly. They found that AbiEi binds to AbiEii to form a stable toxin-antitoxin complex in solution. Importantly, antitoxin binding leads to AbiEii auto-phosphorylation, a modification in which the toxin adds a phosphate group to itself, and this modification abolishes the NTase activity of AbiEii. Based on these findings, the team reclassified AbiE systems as Type VII toxin-antitoxin systems rather than the previously reported Type IV class. The experiments further showed that toxin phosphorylation is dynamic and reversible: the sole serine/threonine phosphatase Sa STP of S. agalactiae can efficiently dephosphorylate AbiEii in vitro. Mutagenesis studies and functional assays supported a widespread mechanism in which toxin auto-phosphorylation accounts for AbiEii neutralization, and these data were presented as supporting recent hypotheses about control of homologous MenAT toxin-antitoxin systems from Mycobacterium tuberculosis.

These results change how scientists think about this family of toxin-antitoxin systems. Rather than simple sequestration or degradation, the antitoxin AbiEi actively induces a chemical switch in AbiEii that turns off its enzymatic activity. The discovery that phosphorylation and dephosphorylation — mediated by AbiEi and the phosphatase Sa STP, respectively — control the toxin suggests a reversible regulatory system that can rapidly respond to cellular conditions. Reclassifying AbiE as a Type VII system also reframes comparisons with related systems in other bacteria, and the work explicitly links the mechanism observed here to hypotheses about MenAT systems in Mycobacterium tuberculosis. For researchers studying bacterial physiology, antibiotic resistance, or the biology of pathogens, this study provides a clear molecular example of how a cell neutralizes a growth-inhibiting NTase while preserving global translation and other essential functions.