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

## TL;DR

This study identifies two key enzymes in Pseudomonas aeruginosa that help it make cysteine, a process absent in humans and a potential target for new antibiotics.

## Contribution

The study characterizes two cysteine synthase isoforms in P. aeruginosa and their roles in sulfur assimilation.

## Key findings

- PA2709 and PA0932 are the two main cysteine synthase isoforms in P. aeruginosa.
- Double deletion of these genes is required to achieve cysteine auxotrophy.
- PA0932 shows a cysteine bradytrophic phenotype when thiosulfate is the only sulfur source.

## Abstract

Most bacteria synthesize L-cysteine via 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 Pseudomonas aeruginosa, a major antimicrobial resistance (AMR)-associated pathogen.

We have identified the two main isoforms of cysteine synthase in P. aeruginosa (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 O-acetylserine (OAS) sulfhydrylase, using bisulfide as a sulfur source. PA0932 also shows an OAS-dependent S-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.

•Sulfate assimilation pathway, absent in humans, provides new antibiotic targets•Two such targets are the cysteine synthases PaCysK and PaCysM of P. aeruginosa•The physiological role and substrate specificity of PaCysK and PaCysM were elucidated•Some functions of PaCysK and PaCysM may not be associated with cysteine production

Sulfate assimilation pathway, absent in humans, provides new antibiotic targets

Two such targets are the cysteine synthases PaCysK and PaCysM of P. aeruginosa

The physiological role and substrate specificity of PaCysK and PaCysM were elucidated

Some functions of PaCysK and PaCysM may not be associated with cysteine production

Biosynthesis; Microbial genetics; Molecular microbiology