Fig. 2. Racemase RacK is responsible for d-Lys secretion and PG editing.

(A) Overview of the colorimetric assay used to quantify d-Lys secretion. Purified d-Lys oxidase AmaD from P. putida catalyzes the oxidative deamination of d-Lys to release 6-amino-2-oxohexanoate, hydrogen peroxide (H₂O₂), and ammonium (NH₃). H₂O₂ produced was then quantified by treatment with horseradish peroxidase (HRP) and a fluorophore dye. The amount of fluorescence is converted to d-Lys concentration through a standard curve. (B) Quantification of d-Lys present in supernatant fractions of Ab17978 WT, 17978ΔRacK, and the complemented strain (17978 RacK⁺). Data represent means ± SD of three biological replicates. (C) Chromatograms of purified, stationary phase muropeptides from the indicated strains. Muropeptide structures are shown in fig. S2.