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

The ubiquitous bacterium Caulobacter crescentus holds promise to be used in bioremediation applications due to its ability to mineralize U­(VI) under aerobic conditions. Here, cell free extracts of C. crescentus 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.