De novo cysteine biosynthesis in Pseudomonas aeruginosa: Characterization of the two main cysteine synthase isoforms

Most bacteria synthesize L-cysteine <i>via</i> the reductive sulfate assimilation pathway, which is absent in humans and thus a promising source of antibiotic targets. Despite its relevance, this pathway remains poorly studied in <i>Pseudomonas aeruginosa</i>, a major antimicrobial resistance (AMR)-associated pathogen. We have identified the two main isoforms of cysteine synthase in <i>P. aeruginosa</i> (PA2709 and PA0932), which are pyridoxal 5'-phosphate-dependent enzymes that enable bacterial growth in minimal medium supplemented with either sulfate or thiosulfate. PA2709 is a classical <i>O</i>-acetylserine (OAS) sulfhydrylase, using bisulfide as a sulfur source. PA0932 also shows an OAS-dependent <i>S</i>-sulfocysteine synthase activity. Deletion of either one of the two genes does not lead to cysteine auxotrophy, which is reached only with the double deletion mutant. Interestingly, in the presence of thiosulfate as the only sulfur source, PA0932 displays a cysteine bradytrophic phenotype, suggesting the activation of an alternative sulfur assimilation pathway under these conditions.