System-level understanding of the potential acid-tolerance components of Acidithiobacillus thiooxidans ZJJN-3 under extreme acid stress.

In previous study, two extremely acidophilic strains Acidithiobacillus thiooxidans ZJJN-3 (collection site: bioleaching leachate) and ZJJN-5 (collection site: bioleaching wastewater) were isolated from a typical industrial bio-heap in China. Here, we unraveled the potential acid-tolerance components of ZJJN-3 by comparing the physiological differences with ZJJN-5 under different acid stresses. The parameters used for comparison included intracellular pH (pHin), capsule morphology, fatty acid composition of cell membrane, transcription of key molecular chaperones, H(+)-ATPase activities and NAD(+)/NADH ratio. It was indicated that the acid-tolerance of A. thiooxidans ZJJN-3 was systematically regulated. Capsule first thickened and then shed off along with increased acid stress. Cell membrane maintained the intracellular stability by up-regulating the proportion of unsaturated fatty acid and cyclopropane fatty acids. Meanwhile, the transcription of key repair molecular chaperones (GrpE-DnaK-DnaJ) was up-regulated by 2.2-3.5 folds for ensuring the proper folding of peptide. Moreover, low pHin promoted ZJJN-3 to biosynthesize more H(+)-ATPase for pumping H(+) out of cells. Furthermore, the NAD(+)/NADH ratio increased due to the decreased H(+) concentration. Based on the above physiological analysis, the potential acid-tolerance components of A. thiooxidans ZJJN-3 were first proposed and it would be useful for better understanding how these extremophiles responded to the high acid stress.