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.

毒素-抗毒素（TA）系统在细菌中无处不在，并且在许多情况下都被假定在病原体的毒力中起作用。一些金黄色葡萄球菌菌株携带质粒，该质粒编码与维持质粒有关的高毒性PemIKSa TA系统，但也涉及基因表达的调节。在这里，我们显示了pemIKSa1-Sp TA系统，与质粒编码的PemIKSa同源，实际上存在于p.udseintermedius的每个染色体中然而，该菌株表现出序列异质性。这导致了PemKSa1-Sp毒素的两个长度变异。较短的（96aa）C端截短的毒素在酶学上是无活性的，而全长（112aa）的变体是RNase，尽管对宿主细胞无毒。活性PemKSa-Sp2毒素缺乏毒性的原因是底物特异性提高。该pemISa1-SP抗毒素基因似乎pseudogenized，然而，整个pemIKSa1-SP系统具有转录活性。当假定使用替代的起始密码子生产N端截短的抗毒素时，有五个可能的长度变异。在这里，我们显示出即使是截短的抗毒素也能够与PemKSa-Sp2毒素相互作用并抑制其RNase活性。此外，抗毒素可以使细菌细胞免受过量表达质粒编码的PemKSa毒素的毒性作用。总体而言，我们的数据表明，与有毒质粒编码的PemIKSa TA系统相反，pemIKSa1-Sp在假单胞菌染色体中的位置导致其毒性丧失。有趣的是，PemKSa1-Sp2毒素的保留的RNase活性和N端截短的推定抗毒素的功能性分别表明存在缓解/减轻毒素毒性和保持抗毒素抑制活性的进化压力。