Biomedical subjects

J S Cavet

Publications and source records attributed to J S Cavet.

3 recordsLinked to original sources

PubMed2001

A metallothionein containing a zinc finger within a four-metal cluster protects a bacterium from zinc toxicity.

Zinc is essential for many cellular processes, including DNA synthesis, transcription, and translation, but excess can be toxic. A zinc-induced gene, smtA, is required for normal zinc-tolerance in the cyanobacterium Synechococcus PCC 7942. Here we report that the protein SmtA contains a cleft lined with Cys-sulfur and His-imidazole ligands that binds four zinc ions in a Zn(4)Cys(9)His(2) cluster. The thiolate sulfurs of five Cys ligands provide bridges between the two ZnCys(4) and two ZnCys(3)His sites, giving two fused six-membered rings with distorted boat conformations. The inorganic core strongly resembles the Zn(4)Cys(11) cluster of mammalian metallothionein, despite different amino acid sequences, a different linear order of the ligands, and presence of histidine ligands. Also, SmtA contains elements of secondary structure not found in metallothioneins. One of the two Cys(4)-coordinated zinc ions in SmtA readily exchanges with exogenous metal ((111)Cd), whereas the other is inert. The thiolate sulfur ligands bound to zinc in this site are buried within the protein. Regions of beta-strand and alpha-helix surround the inert site to form a zinc finger resembling the zinc fingers in GATA and LIM-domain proteins. Eukaryotic zinc fingers interact specifically with other proteins or DNA and an analogous interaction can therefore be anticipated for prokaryotic zinc fingers. SmtA now provides structural proof for the existence of zinc fingers in prokaryotes, and sequences related to the zinc finger motif can be identified in several bacterial genomes.

Amino Acid Sequence↗

PubMed2001

Microbial metallothioneins.

Bacterial metallothioneins bind, sequester and buffer excess intracellular zinc. At present, the vast majority of the available experimental data relate to cyanobacterial metallothionein, SmtA, from Synechococcus PCC 7942. SmtA is required for normal resistance to zinc and smtA-mediated zinc resistance has been used as a selectable marker. The imidazole groups of histidine residues, in addition to the thiol groups of cysteine residues, co-ordinate zinc in bacterial metallothioneins. The structure of bacterial metallothionein must facilitate some discrimination between 'adventitious' and 'adventageous' zinc-binding sites such that under excess zinc conditions metal is predominantly scavenged from the former. It remains unclear whether or not bacterial metallothionein also acts as a zinc store that supplies zinc-requiring proteins or if under some conditions it deactivates a subset of proteins via zinc removal. Expression of smtA is induced in response to elevated concentrations of zinc via the action of SmtB. SmtB has some sequence similarity to the arsenic responsive repressor ArsR and genes encoding related proteins are present in many bacterial genomes. Metal perception by SmtB differs from ArsR. The latter contains a characteristic Cys-Val-Cys motif associated with a DNA-binding helix-turn-helix (the ArsR motif), while the former contains metal-binding motifs associated with a carboxyl-terminal alpha-helix that forms the interface between SmtB dimers (the SmtB motif). Some SmtB-ArsR family proteins, including the zinc sensor ZiaR from the cyanobacterium Synechocystis PCC 6803, have the metal-sensory motifs of both SmtB and ArsR. The mechanisms of action, and the features that allow discrimination between different metal ions by these sensors, are discussed.

Amino Acid Sequence↗

PubMed1999

Cobalt-dependent transcriptional switching by a dual-effector MerR-like protein regulates a cobalt-exporting variant CPx-type ATPase.

CoaR associates with and confers cobalt-dependent activation of the coaT operator-promoter. A CoaR mutant (Ser-Asn-Ser) in a carboxyl-terminal Cys-His-Cys motif bound the coaT operator-promoter but did not activate expression in response to cobalt, implicating thiolate and/or imidazole ligands at these residues in an allosteric cobalt binding site. Deletion of 1 or 2 nucleotides from between near consensus, but with aberrant (20 base pairs) spacing, -10 and -35 elements enhanced expression from the coaT operator-promoter but abolished activation by cobalt-CoaR. It is inferred that cobalt effects a transition in CoaR that underwinds the coaT operator-promoter to realign promoter elements. In the absence of cobalt, CoaR represses expression (approximately 50%). CoaR is a fusion of ancestral MerR (mercury-responsive transcriptional activator)- and precorrin isomerase (enzyme of vitamin B(12) biosynthesis)-related sequences. Expression from the coaT operator-promoter was enhanced in a partial mutant of cbiE (encoding an enzyme preceding precorrin isomerase in B(12) biosynthesis), revealing that this pathway "inhibits" coaT expression. Disruption of coaT reduced cobalt tolerance and increased cytoplasmic (57)Co accumulation. coaT-mediated restoration of cobalt tolerance has been used as a selectable marker.

Adenosine Triphosphatases↗