Biology Reference
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sequences at the same time, and this cooperativity appears to be mediated
by protein-induced DNA deformation since no protein-protein intermo-
lecular interactions occurred between ManR monomers in vitro (
Huang
& Wu, 2005
). Although MntH transporters of Mn
2+
have been described
in several bacterial groups, the regulation of its expression by the two-
component signal transduction system ManS/ManR has only been found
in cyanobacteria (
Huang & Wu, 2004a
).
Another Mn-uptake regulation mechanism described in
Synechocystis
PCC 6803, different from the more thoroughly characterized ManS/ManR
two-component signal transduction system, involves the RfrA regulator
(
Chandler et al., 2003
). This protein modulates a second high-affinity Mn
transport system which acts under Mn-sufficient conditions, but the mode
of action of RfrA remains unknown. RfrA has no sequence or structural
similarities to previously described bacterial manganese-regulated tran-
scription factors, and it does not have any known DNA-binding domain.
Hence, it is more plausible that RfrA regulates the second Mn
2+
transporter
through a mechanism other than transcriptional control, such as revers-
ible protein modifications at post-translational level. The regulator contains
a conservative repeated-five residues (RFR) domain in the N-terminal,
which define a 16-member family in
Synechocystis
PCC 6803. Despite the
fact that RFR domains seem to be relatively abundant in other bacterial
genomes and especially in photosynthetic organisms, the RFR genes have
no defined function (
Bateman, Murzin, & Teichmann, 1998
). Thus, RfrA
becomes the first member of this family of proteins to be linked to a physi-
ological process (
Chandler et al., 2003
). Further experiments are required to
discern the exact mode of RfrA regulation in Mn uptake in cyanobacteria.
3.4. The ArsR/SmtB Family of Metal-Sensor Proteins
The SmtB/ArsR proteins function as transcriptional repressors sensing ele-
vated concentrations of different metals not only in cyanobacteria but also
in other prokaryotes. This family contains two subfamilies, one comprising
SmtB and its orthologues, more divergent than the other subfamily which
contains ArsR and its closely related proteins. Similarly, all family mem-
bers usually control the expression of a metallothionein to sequester metal
ions in the cytosol, or an ATPase to export the metal into the periplasm.
Conversely, both groups of proteins mainly differ in the metal-binding site
(
Rensing, 2005
).
This protein family includes zinc sensors (SmtB in
Synechococcus
and
ZiaR in
Synechocystis
), arsenic, antimony and bismuth (ArsR in
E. coli
and
