DIFFERENT RESPONSE OF TWO METALLOTHIONEIN SUBFAMILIES EXPOSED TO CHROMIUM (VI) IN Tetrahymena thermophila

The expression of the heavy-metal-binding proteins, metallothioneins (MTs), are induced by heavy metals such as Cd, Cu, Zn, and Hg. MTs attenuate toxic heavy metals by sequestering them and decreasing their intracellular concentrations. However, Cr(VI) inhibits MT gene transcription in higher eukaryotes. In this study, we found that Cr affect Tetrahymena thermophila proliferation, and the lethal concentration killing 50% of the cell population (LC50) of Cr (VI) and Cr (III) is 80 μM and 3 mM, respectively. Cr (VI) upregulates the expression of CuMTs (MTT2, MTT4), but shows little effect for CdMTs (MTT1, MTT3 and MTT5) transcription in the vegetative growth stage of T. thermophila. Furthermore, upregulated expression of CdMTs gene induced by Cd (II) and Cu (II) was inhibited by Cr(VI). In contrast, upregulated expression of CuMTs gene induced by Cd (II) and Cu (II) has less change under Cr(VI) induction. CdMTs and CuMTs showed different response to Cr(VI) induction. The results further showed that the transcription regulation mechanism of CdMTs and CuMTs is different in T. thermophila.