The QS regulator AphB promotes expression of the AHPND PirA and PirB toxins and may enhance virulence under acidic conditions.

Shrimp acute hepatopancreatic necrosis disease (AHPND) is one of the most devastating diseases to impact the global shrimp farming industry, with a mortality rate of 70 %-100 %. The key virulence factors are a pair of Photorhabdus insect-related (Pir)-like toxins, PirA and PirB. In this study, by using an in vitro transcription and translation assay, we first confirmed that the quorum sensing transcriptional regulator AphB could trigger the expression of its downstream genes after binding to the AphB binding sequence in the promoter region of the pirA/pirB operon. Next, we showed that AphB was essential for the expression of these toxins by using an aphB-deletion mutant (ΔaphB) derived from the AHPND-causing Vibrio parahaemolyticus. Lastly, we discovered that the expression levels of PirA and PirB were up-regulated under acidic conditions (pH 4.5), and further showed that an acidic environment promoted the binding of AphB to the pirAB promoter. We speculate that this was because the acidic environment favored the formation of AphB tetramers, which is important for binding to DNA. Taken together, these findings improve our understanding of the gene regulatory mechanisms of pirA and pirB, and suggest that the pH value of the environment might affect the virulence of AHPND-causing V. parahaemolyticus.