Metalloregulatory proteins and molecular mechanisms of heavy metal signal transduction

This review has considered what is known about the precise chemical mechanisms involved in the signal transduction of heavy metal ions. By reviewing what is known about general modes of signal transduction, we may draw parallels with the detection of and response to metal ions. In all forms of signal transduction, sensors and transducers are required. Yet, it is apparent that each system has unique features which undoubtedly are critical for the specific signal at hand. Within the context of metal-responsive systems, regardless of whether or not the metal ion is being sequestered, directly utilized, removed or otherwise, several examples of specific metalloregulatory proteins have been elucidated and are summarized in Table II. A close inspection of Table II reveals that in most signal transduction pathways for heavy metals, the presence of the metal ion causes a marked change in the nucleic acid binding capacity of the metalloregulatory protein. For example, the presence of iron results in the dissociation of a protein from iron responsive elements, thereby derepressing ferritin translation. In other instances, metal binding allows a metalloregulatory protein to associate with DNA to activate or repress transcription, as with ACE1 and Fur, respectively. In fact, to the authors' knowledge, it appears that all characterized ligand-responsive transcription factors change nucleic acid binding activity upon ligand binding. This change in affinity is a major feature of the mechanism for activation or repression by these receptors. In contrast, the mercuric ion metalloregulatory protein, MerR, operates by an entirely different transduction mechanism. MerR remains bound to its operator sequence in the presence and absence of mercuric ion, with only a slight increase in the dissociation rate constant in the presence of Hg(II). Furthermore, the site of MerR binding to the DNA is in a novel position for a prokaryotic activator, directly between the two sets of recognition sequences for RNA polymerase. Analysis of the protein-DNA interactions and transcriptional activity has demonstrated that MerR forms a complex with RNA polymerase in the absence of Hg(II) that is unstable and transcriptionally repressed. When Hg(II) is present in greater than nanomolar concentrations, a highly active transcription complex is formed at PT and a distortion at the center of the palindromic MerR binding site is detectable. Kinetic analysis has determined that, although no change in the binding of RNAP to PT is apparent, the presence of Hg(II) stimulates the rate of isomerization from the closed to the open transcription complex.(ABSTRACT TRUNCATED AT 400 WORDS)