Characterization of MtnE, the fifth metallothionein member in Drosophila

Metallothioneins (MTs) constitute a family of cysteine-rich, low molecular weight metal-binding proteins which occur in almost all forms of life. They bind physiological metals, such as zinc and copper, as well as nonessential, toxic heavy metals, such as cadmium, mercury, and silver. MT expression is regulated at the transcriptional level by metal-regulatory transcription factor 1 (MTF-1), which binds to the metal-response elements (MREs) in the enhancer/promoter regions of MT genes. Drosophila was thought to have four MT genes, namely, MtnA, MtnB, MtnC, and MtnD. Here we characterize a new fifth member of Drosophila MT gene family, coding for metallothionein E (MtnE). The MtnE transcription unit is located head-to-head with the one of MtnD. The intervening sequence contains four MREs which bind, with different affinities, to MTF-1. Both of the divergently transcribed MT genes are completely dependent on MTF-1, whereby MtnE is consistently more strongly transcribed. MtnE expression is induced in response to heavy metals, notably copper, mercury, and silver, and is upregulated in a genetic background where the other four MTs are missing.