The Salmonella Protein That Amplifies a Defence Signal

Summary. Researchers in the Department of Biological Sciences at the University of Alberta in Canada reported in mBio, published online on 9 October, a previously unknown component of the PhoP/PhoQ (PhoPQ) two-component signalling system in Salmonella enterica. PhoPQ is broadly conserved in bacteria and is a central regulator of Salmonella-specific virulence gene expression. The new component is a small putative lipoprotein found only in Salmonella, which the team named PalA. PalA localises to the cytoplasmic membrane, where it interacts with PhoQ, leading to activation of PhoP and a corresponding reprogramming of the genes that PhoP controls.

Research context (RUO). The effect of PalA on gene expression was entirely dependent on PhoPQ, which the authors interpret as strict specificity. When the bacteria were exposed to cationic antimicrobial peptides, a host-derived signal encountered during infection, expression of the palA gene was induced and PalA strongly activated PhoPQ. Under low pH or low magnesium conditions, palA expression was low and its effect minimal, indicating that its role is specific to the antimicrobial peptide signal. PalA did not stimulate PhoPQ systems from closely related species, suggesting that the Salmonella system has adapted to be activated by it. The authors describe this as an evolutionary adaptation that strengthens Salmonella's virulence gene expression in response to antimicrobial peptides. For peptide researchers, it is a reminder that bacteria do not only resist cationic antimicrobial peptides but can also use them as a cue to adjust their own regulatory programmes.