Shotgun Proteomic Analysis Unveils Survival and Detoxification Strategies by <i>Caulobacter crescentus</i> during Exposure to Uranium, Chromium, and Cadmium

The ubiquitous bacterium <i>Caulobacter crescentus</i> holds promise to be used in bioremediation applications due to its ability to mineralize U­(VI) under aerobic conditions. Here, cell free extracts of <i>C. crescentus</i> grown in the presence of uranyl nitrate [U­(VI)], potassium chromate [Cr­(VI)], or cadmium sulfate [Cd­(II)] were used for label-free proteomic analysis. Proteins involved in two-component signaling and amino acid metabolism were up-regulated in response to all three metals, and proteins involved in aerobic oxidative phosphorylation and chemotaxis were down-regulated under these conditions. Clustering analysis of proteomic enrichment revealed that the three metals also induce distinct patterns of up- or down-regulated expression among different functional classes of proteins. Under U­(VI) exposure, a phytase enzyme and an ABC transporter were up-regulated. Heat shock and outer membrane responses were found associated with Cr­(VI), while efflux pumps and oxidative stress proteins were up-regulated with Cd­(II). Experimental validations were performed on select proteins. We found that a phytase plays a role in U­(VI) and Cr­(VI) resistance and detoxification and that a Cd­(II)-specific transporter confers Cd­(II) resistance. Interestingly, analysis of promoter regions in genes associated with differentially expressed proteins suggests that U­(VI) exposure affects cell cycle progression.