Biology Reference
In-Depth Information
allowing RNA polymerase to initiate transcription from a suboptimal pro-
moter, which has indeed become a potent one. MerR proteins are also
capable of autoregulating their own expression (
Brown, Stoyanov, Kidd, &
Hobman, 2003
). The MerR family includes sensors of mercury (MerR in
transposable elements Tn
21
or Tn
501
), zinc, cadmium and lead (ZntR in
E. coli
), lead (PbrR in
Ralstonia metallidurans
), copper, gold and silver (CueR
in
E. coli
, GolS in
Salmonella
) or cobalt (CoaR in
Synechocystis
) (
Osman &
Cavet, 2010
).
It is noteworthy that MerR proteins also act as weak repressors in the
absence of the metal despite being activators. Both repression and activation
processes occur while MerR proteins are bound at the same DNA region,
between the −10 and −35 promoter elements. This promoter sequence
presents an unusual structure as it is about 2 bp longer (19-20 bp) than
canonical bacterial promoters. It impairs RNA polymerase from function-
ing until DNA bending and twisting is possible by metal-regulator interac-
tion. A high degree of sequence similarity in the N-terminal DNA-binding
region is another important feature among members of the MerR family,
containing a predicted HTH motif followed by a long coiled-coil region.
A small C-terminal domain confers metal selectivity (
Brown et al., 2003
).
In Gram-negative bacteria, MerR regulates expression of the
merTP(C/F)
AD(E)
operon in response to mercury. This metal resistance operon encodes
the putative transporters MerT, MerC, MerF or MerE, a periplasmic protein
MerP, a mercuric reductase MerA and a putative repressor MerD. MerR
proteins from these transposons are 144-amino-acid long with a much
conserved N-terminal sequence and a variable C-terminal region. MerR
family members from Gram-positive bacteria differ from those from Gram-
negative ones; they share about 37% amino acid identity. Nonetheless, all
MerR proteins contain three conserved cysteine residues, suggested to be
involved in Hg
2+
coordination.
In cyanobacteria, an MerR orthologue has been identified by two differ-
ent research groups as CoaR or CorR.
Synechocystis
PCC 6803
coaR
(
corR
)
is divergently transcribed from
coaT
(
corT
), encoding a putative Co
2+
efflux
pump. CoaR is a 370-amino-acid long protein with two different domains.
The N-terminal domain aligns with MerR proteins, while the C-terminal
region shows sequence similarity to precorrin isomerases, involved in the
biosynthetic pathway of cobalamin. This vitamin B
12
contains four corrin
rings coordinating a cobalt atom. Notably, a Cys-His-Cys C-terminal motif
is involved in cobalt sensing. The
coaR-coaT
intergenic region contains a
20-bp spacer with the AAACCTTGCATT-N
6
-AATGTTAAGGTTT
