Atypical thioredoxin Patrx2 enhances alginate production in mucoid Pseudomonas aeruginosa

Pseudomonas aeruginosa, an opportunistic human pathogen, is known for its ability to respond and adapt to its environment, employing intricate adaptation mechanisms that can lead to the formation of complex biofilms. Redox processes play a pivotal role in bacterial adaptation mechanisms. The cytoplasm of most organisms is recognized for maintaining a reducing environment through thiol-disulfide oxidoreductases. In Pseudomonas aeruginosa, we have identified an unusual cytoplasmic thioredoxin named Patrx2. What sets Patrx2 apart is its active site, which contains a consensus sequence, CGHC, identical to the characteristic motif of protein disulfide isomerases (PDIs) found in eukaryotic cells. Our investigations have unveiled that Patrx2, unlike canonical thioredoxins, exhibits disulfide isomerase activity in vitro and displays physicochemical properties, as well as a structural conformation of its catalytic site, reminiscent of PDIs. Using a mutant transposon library, we found that the expression of patrx2 is regulated by the alternative sigma factor AlgU, which is implicated in the formation of alginate biofilms in P. aeruginosa. We further demonstrated strong patrx2 expression in a mucoid strain we constructed, carrying the clinically relevant mucA22 mutation frequently found in cystic fibrosis patients. Furthermore, our results showed a significant decrease in alginate synthesis in a patrx2 mutant in this mucoid strain, this effect was also observed in the C34S catalytic variant, suggesting a role for Patrx2's catalytic site in this phenotype. The study of Patrx2, an atypical thioredoxin expressed within an alginate biofilm, underscores the importance of redox regulation in adaptation mechanisms. The induction of Patrx2's expression in alginate-producing biofilms highlights its potential relevance in redox-regulated adaptation mechanisms.