Thioesterase modulates antibacterial Type VI secretion system via alteration in oxidative stress responses in Burkholderia

The Type VI secretion system (T6SS) is a specialized protein secretion mechanism used by bacteria to dominate interbacterial competition and kill prey bacteria. In a recent study, researchers found that the disruption of the thioesterase gene, encoded in the gramibactin biosynthetic gene cluster, activates the T6SS in the Burkholderia gladioli strain NGJ1, even without the presence of prey bacteria.

The thioesterase mutant demonstrated increased antibacterial activity, and the complementation of this gene restored the original phenotype. Strikingly, both the thio mutant and its complement showed heightened sensitivity to oxidative stress. To investigate further, researchers employed quantitative reverse transcription PCR (qRT-PCR) analysis, which revealed that the thio mutant exhibited upregulated expression of RpoN1, a sigma factor that positively regulates T6SS functions.

In contrast, the rpoN1 mutant was hypersensitive to oxidative stress and defective in T6SS activation, while overexpression of RpoN1 enhanced oxidative stress tolerance in NGJ1 and activated T6SS. The researchers propose that, during interbacterial conflicts encountered during plant colonization, the NGJ1 strain encounters an oxidative-stress-enriched environment that modulates thioesterase expression, induces RpoN1, and subsequently activates the T6SS.

The study highlights the broader physiological role of thioesterase in maintaining intracellular reactive oxygen species (ROS) homeostasis in bacteria and driving antibacterial activity.