The PinT/PiaT novel toxin-antitoxin system affecting prophage induction in Pseudomonas aeruginosa

Prophages are regions in bacterial genomes that have originated from lysogenic viral infections. Recent research showed that in a pool of 2110 bacterial genomes, almost half of the genomes contain such regions. Among Pseudomonas aeruginosa strains, the prevalence of prophages is even higher; at least one prophage region was found in every strain analyzed. Prophages may undergo induction to the lytic cycle and start to produce infective virions due to environmental or internal triggers.

Pseudomonas aeruginosa PAO1 reference strain, encompass two prophages including the Pf4 phage. This ~12 Kbp phage contains two ORF that shows homology to a Toxin-Antitoxin (TA) system. TA systems are small genetic modules consisting of two elements – a toxin and an antitoxin, to counteract the activity of the toxic protein. Various functions have been assigned to TA systems, including persistence, DNA stabilization, protection against mobile genetic elements and more. Here we describe and characterize this TA system named PinT/PiaT, with a novel function of lytic infection induction. These two genes constitute a type II TA system wherein the antitoxin PiaT binds to the PinT toxin, thus inhibiting its toxicity, similarly to a previously described hybrid TA pair ParE/YefM in the phytopathogen Erwinia pyrifoliae. In addition, our finding suggests that absence of PiaT leads to increased virion production. The results of this study showed that PinT and PiaT have the characteristics of TA system with a novel function of phage induction.