Chromosomal localization of PemIK toxin-antitoxin system results in the loss of toxicity - Characterization of pemIK-Sp from Staphylococcus pseudintermedius.

Toxin-antitoxin (TA) systems are ubiquitous in bacteria and on numerous occasions have been postulated to play a role in virulence of pathogens. Some Staphylococcus aureus strains carry a plasmid, which encodes the highly toxic PemIKSa TA system involved in maintenance of the plasmid but also implicated in modulation of gene expression. Here we showed that pemIKSa1-Sp TA system, homologous to the plasmid-encoded PemIKSa, is present in virtually each chromosome of S. pseudintermedius strain, however exhibits sequence heterogeneity. This results in two length variants of the PemKSa1-Sp toxin. The shorter (96 aa), C-terminally truncated toxin is enzymatically inactive, whereas the full length (112 aa) variant is an RNase, though nontoxic to the host cells. The lack of toxicity of the active PemKSa-Sp2 toxin is explained by increased substrate specificity. The pemISa1-Sp antitoxin gene seems pseudogenized, however, the whole pemIKSa1-Sp system is transcriptionally active. When production of N-terminally truncated antitoxins using alternative start codons is assumed, there are five possible length variants. Here we showed that even substantially truncated antitoxins are able to interact with PemKSa-Sp2 toxin and inhibit its RNase activity. Moreover, the antitoxins can rescue bacterial cells from toxic effects of overexpression of plasmid-encoded PemKSa toxin. Collectively, our data indicates that, contrary to the toxic plasmid-encoded PemIKSa TA system, location of pemIKSa1-Sp in the chromosome of S. pseudintermedius results in the loss of its toxicity. Interestingly, the retained RNase activity of PemKSa1-Sp2 toxin and functionality of the putative, N-terminally truncated antitoxins suggest the existence of evolutionary pressure for alleviation/mitigation of the toxin's toxicity and retention of the inhibitory activity of the antitoxin, respectively.