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, tra nscription, and translation, but excess can be toxic. A zinc-induced gene, smtA, is required for normal zinc-tolerance in the cyanobacterium Synechoco ccus 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 b ridges between the two ZnCys(4) and two ZnCys(3)His sites, giving two fused six-membered rings with distorted boat conformations. The inorganic core s trongly resembles the Zn(4)Cys(11) cluster of mammalian metallothionein, de spite different amino acid sequences, a different linear order of the ligan ds, and presence of histidine ligands. Also, SmtA contains elements of seco ndary structure not found in metallothioneins. One of the two Cys(4)-coordi nated zinc ions in SmtA readily exchanges with exogenous metal (Cd-111), wh ereas 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 -hel ix surround the inert site to form a zinc finger resembling the zinc finger s in GATA and LIM-domain proteins. Eukaryotic zinc fingers interact specifi cally 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 relat ed to the zinc finger motif can be identified in several bacterial genomes.