Functional diversification despite structural congruence in the HipBST toxin-antitoxin system ofLegionella pneumophila

Abstract

ABSTRACTToxin-antitoxin (TA) systems are abundant genetic modules in bacterial chromosomes and on mobile elements. They are often patchily distributed and their physiological functions remain poorly understood. Here, we characterize a TA system inLegionella pneumophilathat is highly conserved acrossLegionellaspecies. This system is distantly related toEscherichia coliHipBST and we demonstrate that it is a functional tripartite TA system (denoted HipBSTLp). We identify HipBSTLphomologs in diverse taxa, yet in the Gammaproteobacteria these are almost exclusively found inLegionellaspecies. Notably, the toxin HipTLpwas previously reported to be a pathogenic effector protein that is translocated byL. pneumophilainto its eukaryotic hosts. Contrary to this, we find no signal of HipTLptranslocation beyond untranslocated control levels and make several observations consistent with a canonical role as a bacterial toxin. We present structural and biochemical insights into the regulation and neutralization of HipBSTLp, and identify key variations between this system and HipBSTEc. Finally, we show that the target of HipTLpis likely not conserved with any characterized HipA or HipT toxin. This work serves as a unique comparison of a TA system across bacterial species and illustrates the molecular diversity that exists within a single TA family.