Analysis of Gene Expression Provides Insights into the Mechanism of Cadmium Tolerance in Acidithiobacillus ferrooxidans

Acidithiobacillus ferrooxidans plays a critical role in metal solubilization in the biomining industry, and occupies an ecological niche characterized by high acidity and high concentrations of toxic heavy metal ions. In order to investigate the possible metal resistance mechanism, the cellular distribution of cadmium was tested. The result indicated that Cd2+ entered the cells upon initial exposure resulting in increased intracellular concentrations, followed by its excretion from the cells during subsequent growth and adaptation. Sequence homology analyses were used to identify 10 genes predicted to participate in heavy metal homeostasis, and the expression of these genes was investigated in cells cultured in the presence of increasing concentrations of toxic divalent cadmium (Cd2+). The results suggested that one gene (cmtRA.f) encoded a putative Cd2+/Pb2+-responsive transcriptional regulator; four genes (czcA1A.f, czcA2A.f, czcB1A.f; and czcC1A.f) encoded heavy metal efflux proteins for Cd2+; two genes (cadA1A.f and cadB1A.f) encoded putative cation channel proteins related to the transport of Cd2+. No significant enhancement of gene expression was observed at low concentrations of Cd2+ (5 mM) and most of the putative metal resistance genes were up-regulated except cmtRA.f, cadB3A.f; and czcB1A.f at higher concentrations (15 and 30 mM) according to real-time polymerase chain reaction. A model was developed for the mechanism of resistance to cadmium ions based on homology analyses of the predicted genes, the transcription of putative Cd2+ resistance genes, and previous work.