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

Toxin-antitoxin (TA) systems are abundant genetic modules in bacterial chromosomes and on mobile elements. They are often patchily distributed, as a consequence of horizontal exchange, and the breadth of their functionality in bacterial physiology remains unknown. Here, we characterize a TA system in Legionella pneumophila that is highly conserved across Legionella species. This system is distantly related to Escherichia coli HipBST, and we demonstrate that it is a functional tripartite TA system (denoted HipBST(Lp)). We identify HipBST(Lp) homologs in diverse taxa, yet in the Gammaproteobacteria, these are almost exclusively found in Legionella species. Notably, the toxin HipT(Lp) was previously reported to be a pathogenic effector protein that is translocated by L. pneumophila into its eukaryotic hosts. Contrary to this, we find no signal of HipT(Lp) translocation 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 HipBST(Lp), and identify key variations between this system and HipBST(Ec). Finally, we show that the target of HipT(Lp) is likely not conserved with any characterized HipA or HipT toxin. This work serves as a useful comparison of a TA system across bacterial species and illustrates the molecular diversity that exists within a single TA family. IMPORTANCE: Toxin-antitoxin (TA) systems are parasitic genetic elements found in almost all bacterial genomes. They are exchanged horizontally between cells and are typically poorly conserved across closely related strains and species. Here, we report the characterization of a tripartite TA system in the bacterial pathogen Legionella pneumophila that is highly conserved across Legionella species genomes. This system (denoted HipBST(Lp)) is a distant homolog of the recently discovered split-HipA system in Escherichia coli (HipBST(Ec)). We present bioinformatic, molecular, and structural analyses of the divergence between these two systems and the functionality of this newly described TA system family. Furthermore, we provide evidence to refute previous claims that the toxin in this system (HipT(Lp)) possesses bifunctionality as an L. pneumophila virulence protein. Overall, this work expands our understanding of the split-HipA system architecture and illustrates the potential for undiscovered biology in these abundant genetic elements.