Investigation of cobalt bioaccumulation in Rhodopseudomonas palustris by proteomics analysis

Cobalt bioaccumulation by Rhodopseudomonas palustris (R. palustris), which has different modes of metabolism for tolerating heavy metal stress, was investigated using proteomics analysis under aerobic-dark conditions in the presence of a high cobalt concentration. The metal tolerance stress of R. palustris was investigated and compared with the proteome of Co2+-exposed cells and cells grown in the control medium by two-dimensional (2-D) gel electrophoresis and mass spectrometry analysis. Significant changes in the expression level were detected for 36 proteins from the 2-D electrophoresis spectrum. Among them, 25 proteins were identified by MALDI TOF MS/MS, of which 19 were up-regulated and 6 were down-regulated, and they were divided into 7 categories based on their function, including protein synthesis, protein proteolysis, stress protein, carbohydrate metabolism, amino acid metabolism, transport protein and other proteins. The up-regulated mRNA expression of methionine adenosyltransferase (MAT) and phosphate ABC transporter substrate-binding protein (PSTS) was confirmed by semi-quantitative RT-PCR and quantitative RT PCR (qRT-PCR). The mRNA expression of Mat and Psts, which belong to the general category of amino acid metabolism for antioxidation and free phosphate radical transportation protein, increased 4.03-fold and 12.99-fold, respectively, in the process of cobalt bioaccumulation. These results indicate that the up-regulated MAT and PSTS are the key enzymes in the cobalt stress response in the metabolic process, and serve as a positive correlation between the content of the proteins and their mRNA levels.