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Toxin-antitoxin (TA) systems are small genetic modules usually consisting of two elements-a toxin and an antitoxin. The abundance of TA systems among various bacterial strains may indicate an important evolutionary role. Pseudomonas aeruginosa, which can be found in a variety of niches in nature, is an opportunistic pathogen for various hosts. While P. aeruginosa strains are very versatile and diverse, only a few TA systems were characterized in this species. Here, we describe a newly characterized TA system in P. aeruginosa that is encoded within the filamentous Pf4 prophage. This system, named PfiT/PfiA, is a homologue of the ParE/YefM TA system. It is a type II TA system, in which the antitoxin is a protein that binds the toxic protein and eliminates the toxic effect. PfiT/PfiA carries several typical type II characteristics. Specifically, it constitutes two small genes expressed in a single operon, PfiT inhibits growth and PfiA eliminates this effect, PfiA binds PfiT, and PfiT expression results in elongated cells. Finally, we assigned a novel function to this TA system, where an imbalance between PfiT and PfiA, favouring the toxin, resulted in cell elongation and an increase in virion production.

Toxin-antitoxin (TA) systems are small genetic modules usually consisting of two elements-a toxin and an antitoxin. The abundance of TA systems among various bacterial strains may indicate an important evolutionary role. Pseudomonas aeruginosa, which can be found in a variety of niches in nature, is an opportunistic pathogen for various hosts. While P. aeruginosa strains are very versatile and diverse, only a few TA systems were characterized in this species. Here, we describe a newly characterized TA system in P. aeruginosa that is encoded within the filamentous Pf4 prophage. This system, named PfiT/PfiA, is a homologue of the ParE/YefM TA system. It is a type II TA system, in which the antitoxin is a protein that binds the toxic protein and eliminates the toxic effect. PfiT/PfiA carries several typical type II characteristics. Specifically, it constitutes two small genes expressed in a single operon, PfiT inhibits growth and PfiA eliminates this effect, PfiA binds PfiT, and PfiT expression results in elongated cells. Finally, we assigned a novel function to this TA system, where an imbalance between PfiT and PfiA, favouring the toxin, resulted in cell elongation and an increase in virion production.

Surface Acoustic Waves (SAW) were previously shown to inhibit biofilm formation, increase bacterial susceptibility to antibiotic treatment and alter the transcription pattern of Pseudomonas aeruginosa. Here we characterize one gene, sawR (PA3133), that is highly overexpressed when P. aeruginosa is exposed to SAW. SawR is a putative transcription factor belonging to the TetR regulator family. When overexpressed sawR causes numerous phenotypes, including the accumulation of a brown pigment which we identified as pyomelanin. In this study we describe how sawR regulates pyomelanin synthesis. We show that sawR down-regulates the expression levels of hmgA and this causes the accumulation of homogentisic acid which in turn undergoes oxidation and polymerization to pyomelanin. Using bioinformatics, we were able to identify a specific amino acid, arginine 23, which is found within the sawR DNA binding domain and is crucial for its regulatory activity. Our results indicate that sawR does not affect any other genes in the phenylalanine/tyrosine metabolic pathway and its repressive ability on hmgA is not mediated by the hmgA repressor PA2010 (i.e. hmgR). Taken together, our results shed light on the regulatory cascade controlling pyomelanin synthesis and uncover yet another unknown regulator involved in its regulation.