[Function comparison and evolution analysis of metallothionein gene MTT2 and MTT4 in Tetrahymena thermophila]

Tetrahymena has a high genetic polymorphism of metallothionein proteins. These protein isoforms can be divided into subfamilies 7a and 7b. We used real-time quantitative PCR to test the expression levels of Tetrahymena thermophila metallothionein genes MTT2 and MTT4 after exposure to different inducers, including Hg, Cu, Cd, Zn and H(2)O(2). Both genes were most efficiently induced by Cu and secondly by Hg. Their expression was slightly up-regulated after exposure to Cd and Zn, but down-regulated by H(2)O(2). The expression pattern differed from those reported in the 7a subfamily, but was consistent with 7b subfamily members. However, the induced expression level of MTT4 was much higher than that of MTT2, which might be due to differences in their upstream regulatory elements (AP-1, MRE). The high similarities in gene structures and functions indicate that MTT2 and MTT4 were generated by recent gene duplication following the dosage balance model.