A Small Molecule Inhibitor of CTP Synthetase Identified by Differential Activity on a Bacillus subtilis Mutant Deficient in Class A Penicillin-Binding Proteins

In the course of screening for compounds with differential growth inhibition activity on a mutant of Bacillus subtilis lacking all four class A penicillin-binding proteins (Δ4), we came across an isoquinoline derivative, IQ-1 carboxylic acid (IQC) with relatively high activity on the mutant compared to the wild type strain. Treated cells were slightly elongated and had altered chromosome morphology. Mutants of Δ4 resistant to IQC were isolated and subjected to whole genome sequencing. Most of the mutants were affected in the gene, pyrG, encoding CTP synthetase (CTPS). Purified wild type CTPS was inhibited in vitro by IQC. Two of the three mutant proteins purified showed decreased sensitivity to IQC in vitro. Finally, inhibition by IQC was rescued by addition of cytidine but not uridine to the growth medium, consistent with the notion that IQC acts by reducing the synthesis of CTP or a related compound. IQC provides a promising new starting point for antibiotic inhibitors of CTPS.