Biology Reference
In-Depth Information
inverted repeat sequence. CoaR binds to this DNA region, and it responds
to Co
2+
and to Zn
2+
to a lesser extent. There are two proposed activation
models. The first, described by García-Domínguez et al., suggests that Co
2+
binds the corrinoid ring and this complex interacts with CoaR to activate
it. Conversely, Rutherford et al. propose that the metal and the corrinoid
ring bind to different domains in CoaR (
García-Dominguez, Lopez-Maury,
Florencio, & Reyes, 2000
;
Rutherford, Cavet, & Robinson, 1999
).
3.6. The Nickel-Sensor Proteins
Another important family of metal-sensor proteins is made up of NikR
orthologues. In general, NikR proteins act as repressors when bound to
DNA in the presence of the co-repressor metal nickel. However, nickel-
bound
H. pylori
NikR can also function as a DNA activator. NikR pro-
teins repress the transcription of the
nik
operon, encoding the nickel
uptake transporter NikABCDE (
Osman & Cavet, 2010
). Neither NikR
orthologues nor Ni(II)-dependent SmtB/ArsB family members have been
identified in cyanobacteria to date.
3.6.1. Nickel-sensing systems in cyanobacteria
Nickel sensing has been best investigated in
Synechocystis
PCC 6803, where two
kinds of mechanisms are involved, one controlled by a two-component system
that detects periplasmic nickel while the other consists of a cytosolic nickel sensor.
Some years ago, García-Domínguez et al. discovered a metal-regulated
cluster in
Synechocystis.
This includes the previously described zinc and
cobalt response system ZiaR-ZiaA and CoaR-CoaT, respectively (
Fig. 4.7
).
Figure 4.7
Genetic organization of the metal-regulated cluster in
Synechocystis
. The
different metal-resistance operons and target DNA sequences for nickel resistance are
indicated. Genes encoding metalloregulators (dotted arrows), ATPase genes (light grey
arrows), genes encoding the two-component system NrsRS (dark grey arrows) and
genes with unknown function (white arrows) represented. Direct or inverted repeat
sequences present in the NrsR and InrS DNA-binding sites are denoted with arrows
(
Foster et al., 2012
). See the colour plate.
