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

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 HipBSTLp) is a distant homolog of the recently discovered split-HipA system in Escherichia coli (HipBSTEc). 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 (HipTLp) 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.

HipBST Toxin-antitoxin System of Prokaryotes / 中国生物化学与分子生物学报

Toxin-antitoxin system (TA) is a genetic element widely found in chromosomes and plasmids of bacteria, archaea and prophages. TA usually consists a toxin that inhibits the growth of bacteria and an antitoxin that neutralizes its toxicity. Since the discovery of the first CcdB / CcdA TA in the 1980s, TA has been proved to exist in almost all sequenced microorganisms and plays an important role in maintaining plasmid stability, anti-phage and promoting biofilm formation. At present, TA is divided into type I-VIII, among which type IITA is the most widely studied. HipBA is a type II TA. The toxin HipA in Escherichia coli HipBA is a serine / threonine kinase, which inhibits protein translation by phosphorylating bacterial Glutamyl tRNA synthetase (GltX), and its toxicity can be specifically neutralized by HipB. Recently, it has been found that Escherichia coli HipA homologous proteins exist widely in microorganisms, and they form a potential novel TA with genes of the same promoter, in which HipBST has been confirmed by experiments. The toxin HipT and the antitoxin HipS in this TA are similar to the C-terminal and N-terminal of E. coli HipA respectively, and the neutralization mechanism and the substrate of the toxin are different from that of E. coli toxin HipA. This study summarizes the recent discovery of special TA, especially the neutralization mechanism of HipBST which widely exists in prokaryotes.