Metal Ions as Effector Molecules

Metal ions can serve as effector molecules as well as control the DNA-binding activity of regulatory proteins. An example is the regulation of the metallothionein gene in eucaryotes (Fig. 1.23). The metallothioneins are small, cysteine rich proteins which can specifically bind metal ions like Cu or Zn The complexation of metal ions functions to sequester the ions in a form that is not damaging to the cell. 

The gene for metallothionein is induced by metal ions. The goal of the regulation is to provide enough metallothionein for the complexation of metal ions and thus to maintain the concentration of free metal ions at a tolerable level for the cell. 

The transcription of genes for metallothionein is imder positive regulation by the DNA-binding protein ACE, which binds an ACE-specific DNA element in the metal- 

A recent example for the regulation of DNA-binding proteins by metal ions is the transcriptional repressor DREAM that binds to the cognate DNA element only in the absence of Ca (Carrion et al., 1999). An increase of Ca in the form of a Ca signal (see Chapter 6) leads to reduced affinity of the repressor for its DNA element and to an increased expression of the target gene. 

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