Biology Reference
In-Depth Information
their characterization has contributed significantly to providing important
insights into how bacteria handle metals and harmonize many aspects of
their metabolism. The high sensitivity and selectivity of metalloregulatory
proteins has been explained on a molecular basis by solving the crystal
structures of representatives from the main families involved in this impor-
tant function (
Cook, Kar, Taylor, & Hall, 1998
;
Pohl et al., 2003
).
2. GENERAL FEATURES OF METALLOREGULATORS
Metal-sensor proteins are usually allosteric proteins whose revers-
ible interaction with the regulatory metal drives conformational changes
that affect DNA binding. They exhibit metal-binding and DNA-binding
domains and, in some cases, possess an additional structural metal (
Dian
et al., 2011
;
Lucarelli et al., 2007
). Usually, DNA-binding proteins act as a
dimer or a dimer of dimers. In some cases, metal sensors with different metal
specificities may interact (
Fleischhacker & Kiley, 2011
;
Reyes-Caballero,
Campanello, & Giedroc, 2011
).
Residues involved in metal binding are highly conserved. Some metal-
sensor proteins can also be involved in other functions apart from transcrip-
tional regulation. This potential multifunctionality of metalloregulators is a
challenging issue for future investigation aimed at better understanding of
cell physiology.
3. MAIN FAMILIES OF METALLOREGULATORS
IN CYANOBACTERIA
3.1. The Fur Superfamily
Ferric Uptake Regulator (Fur) proteins are among the most ubiquitous
regulators in prokaryotic organisms. Initially identified as iron-sensing
repressors in heterotrophic bacteria, Fur was the first member character-
ized and the protein that gives its name to the family. It is assumed that Fur
works as a classical repressor using Fe
2+
as a cofactor to negatively regulate
expression of their target genes. However, it is now known that Fur proteins
display an ample diversity in metal selectivity and biological functions (
Lee
& Helmann, 2007
), including sensors of iron (Fur), zinc (Zur), manganese
(Mur) and nickel (Nur). Other family members use metal-catalysed oxida-
tion reactions to sense peroxide stress (PerR) or the availability of haeme
(Irr). Thus, Fur proteins are involved in the control of a large number of
