ANALYSIS OF FUNCTIONAL COMPENSATION OF TWO DIFFERENT METALLOTHIONEIN SUBFAMILIES FROM TETRAHYMENA THERMOPHILA

Metallothioneins in Tetrahymena thermophila were divided into two subfamilies: 7a including MTT1, MTT3 and MTT5 and 7b including MTT2 and MTT4. To explore whether these two subfamilies have functional compensation, the MTT1-MTT3 and MTT2-MTT4 gene knockout vectors were constructed. MTT1-MTT3 and MTT2-MTT4 somatic knockout strains were obtained by homologous recombination. We discovered that △MTT1-MTT3 mutant strains were more sensitive to Cd2+ compared with the wild-type strain, and △MTT2-MTT4 mutant strains were more sensitive to Cu2+ and H2O2 compared with the wild-type strain. The diminished expression of MTT5 was observed in △MTT2-MTT4 strains, and 500 mol/L Cu2+ treatment upregulated the expression of MTT1, MTT3 and MTT5 in △MTT2-MTT4 strains. The expression level of MTT2, MTT4 and MTT5 decreased in △MTT1-MTT3 mutant strains, and the treatment of 5 mol/L Cd2+ further diminished the expression level of MTT2 and MTT4 by 4.9 and 2.5 times, respectively, but the treatment of 5 mol/L Cd2+ induced the expression of MTT5. These results demonstrated that MTT1, MTT3 and MTT5 were mainly involved in detoxification, MTT2 and MTT4 were mainly participated in normal cellular metabolism. These two different MTT subfamily proteins are interactional and have functional compensation in Tetrahymena.