Regulation of metallothionein (MT) in Tetrahymena:: Induction of MT-mRNA and protein by cadmium exposure

Cd-dependent regulation of MT expression in Tetrahymena pigmentosa and T. pyriformis was investigated by measuring MT-mRNA and MT accumulation in response to chronic exposure to subtoxic doses of the metal. MT synthesis occurs very soon and MT content increases progressively with Cd accumulation in both species. The induction of MT-mRNA in response to Cd is very rapid, with no observable lag period. At the beginning of exposure, very small amounts of intracellular Cd are sufficient to induce a strong response. Maximum levels of MT-mRNA induction, more than 40-fold, occurred within 30 min of metal treatment in T. pigmentosa. This maximum induction was then followed by down-regulation to 20-fold above the original basal level over the next 30 min. In T. pyriformis, the MT-mRNA maximum level was reached after 60 min, followed by a slow decrease to about 25-fold the basal level within 24 h. This transient fluctuation of mRNA levels is similar to those in other organisms. The data emphasise the importance of an efficient detoxification pathway by complex Cd regulation of MT transcription, which enables Tetrahymena to survive in the continued elevated presence of toxic ions in the environment.