Y. pestis strain 201 and mutant ΔyscI cultures grown under Mh (37°C, TMH with no calcium) condition were used as the experimental and control groups, respectively. After 8 h of incubation, the bacterial pellets were collected by centfugation, re-suspended in lysis buffer (8 M urea in PBS, pH 7.4) and lysed by sonication in an ice bath. The concentrations of proteins were determined using a Pierce BCA Protein Assay Kit. Tandem mass tag labeling was used to obtain accurate quantitative information to minimize variability between samples. For quantitative analysis, at least two unique peptides were used for the identification and quantification of proteins.