Investigation of the basis for Ni tolerance conferred by the expression of TjZnt1 and TjZnt2 in yeast strains

Introduction of ZIP family transporter gene homologues TjZnt1 and TjZnt2 (metal ion transporters) into yeast strains conferred increased Ni(II) tolerance in that species. The action of ZIP family transporter homologues, however, could not explain the Ni resistance of yeast strains transformed with TjZnt1 and TjZnt2. To elucidate the mechanism of Ni tolerance conferred by TjZnt1 and TjZnt2 in yeast strains, we made a series of investigations based upon three hypotheses, including (1) cellular Ni efflux, (2) exclusion of Ni due to competitive uptake of other metals, and (3) Ni binding to histidine-rich domains (chelation). The critical Ni tolerance level of TjZnt2 expressing yeast strains was 1.4 mM, whereas, the TjZnt1 expressing yeast strains were tolerant of Ni concentrations as high as 2.0 mM. The TjZnt1 expressing yeast strain had significantly lower Ni content and significantly higher Zn content than the control and TjZnt2 expressing yeast strain. Effects of the deletion of histidine-rich domain HRD1 or HRD2, or deletion of the region from HRD1 to HRD2, resulted in the same or slightly less Ni(II) tolerance in the TjZnt1 expressing yeast strain. These data indicate that Ni tolerance of the TjZnt2 expressing yeast strain is not correlated with binding to HRDs (Hypothesis 3). Ni tolerance of TjZnt1 expressing yeast strain was, however, partially correlated with Zn influx, which suppressed Ni influx, therefore Ni influx (Hypothesis 1) and competitive inhibition of Ni influx by other metals (Hypothesis 2), remain viable hypotheses which will be subject to further testing.