METAL-RESPONSIVE ELEMENTS OF THE RAINBOW-TROUT METALLOTHIONEIN-B GENE-FUNCTION FOR BASAL AND METAL-INDUCED ACTIVITY

In this study, the contributions of the two metal-responsive elements (MREs) of the rainbow trout (Salmo gairdnerii) metallothionein (tMT)-B gene promoter (-137 to +5) were analyzed. The effect of MRE mutations on the basal and zinc-induced activities of tMT-B promoter-reporter g ene fusions were determined by transfection of a rainbow trout hepatom a (RTH-149) cell line. Together, MREa and MREb cooperate to elicit a s ignificant response to zinc but exhibit differential basal and metal-i nduced activity. The MREa sequence (-62 to -51) is important for basal promoter activity and can function independently, whereas the more di stal MREb (-89 to -100) mainly contributes to metal induction through cooperative interactions with MREa. The degree of basal character of t he MREs is partially determined by nucleotide differences at the flexi ble position N of the MRE consensus TGC(G/A)CNC. In mouse L and HepG2 cells, MREa activity is conserved, but the contributions of the MREb r egion differ, including reduced cooperativity with MREa. There are als o differences in the apparent molecular masses of the rainbow trout an d mammalian nuclear factors that bind to the tMT-B promoter and MREa s equence.