Interplay Between Expression of Sulfur Assimilation Pathway Genes and Zn(2+) and Pb(2+) Stress in Acidithiobacillus ferrooxidans.

We have previously demonstrated that in Acidithiobacillus ferrooxidans, resistance to the highly toxic divalent cation Cd2+ is mediated in part by the sulfur assimilation pathway (SAP) and enhanced intracellular concentrations of cysteine and glutathione(GSH) (Zheng et al., Extremophiles 19:429–436, 2015). In this paper, we investigate the interplay between Zn2+ and Pb2+ resistances, SAP gene expression, and thiol-containing metabolite levels. Cells grown in the presence of 300 mM Zn2+ had enhanced activities of the following enzymes: adenosylphosphosulphate reductase (APR, 40-fold), serine acetyltransferase (SAT, 180-fold), and O-acetylserine (thiol) lyase (OAS-TL, 230-fold). We investigated the concentrations of mRNA transcripts of the genes encoding these enzymes in cells grown in the presence of 600 mM Zn2+: transcripts for 4 SAP genes—ATPS(ATP sulphurylase), APR, SiR(sulfite reductase), SAT, and OAS-TL—each showed a more than three-fold increase in concentration. At the metabolite level, concentrations of intracellular cysteine and glutathione (GSH) were nearly doubled. When cells were grown in the presence of 10 mM Pb2+, SAP gene transcript concentrations, cysteine, and GSH concentrations were all decreased, as were SAP enzyme activities. These results suggested that Zn2+ induced SAP pathway gene transcription, while Pb2+ inhibited SAP gene expression and enzyme activities compared to the pathway in most organisms. Because of the detoxification function of thiol pool, the results also suggested that the high resistance of A. ferrooxidans to Zn2+ may also be due to regulation of GSH and the cysteine synthesis pathway.