Chemistry Reference
In-Depth Information
FIGURE 8.4
Mechanism of RyhB during iron starvation. (A) In conditions of sufficient iron, the activated protein Fur represses RyhB
transcription and (B) the translation of non-essential iron-using proteins is possible. When iron becomes scarce, (1) Fur becomes inactive and
RyhB is rapidly expressed. (2) RyhB is stabilised by the RNA chaperone Hfq. (3) The sRNA RyhB pairs with an mRNA target in an antisense
manner, which blocks translation. (4) The multi-protein complex RNA degradosome recognises the sRNA
e
mRNA target complex and
degrades both RNAs simultaneously.
bacterium Enterococcus hirae. The four genes involved, copY, copZ, copA,andcopB are arranged in the cop
operon. As mentioned in Chapter 7, the proteins CopA and CopB are copper-transporting ATPases, copY is
a copper-responsive repressor, and copZ is a chaperone which is used in intracellular copper metabolism (more of
copper chaperones later in the chapter). The cop operon enables E. hirae to grow in copper-limiting conditions,
as well as in copper concentrations up to 8 mM. Under low-copper conditions, CopA allows Cu acquisition, while
CopB extrudes excess Cu and also Ag.
Fig. 8.5
shows a model for copper homeostasis in E. hirae. Copper enters
the cell via CopA or by nonspecific leakage. Excess cytoplasmic copper binds to CopZ, which can then donate
Cu
þ
to CopB for export and to the CopY repressor to induce the cop operon. In low-copper conditions, two CopY
dimers in the zinc form are bound to the two cop boxes in front of the cop operon. When CopZ donates Cu
þ
to
CopY, one Zn
2
þ
per CopY monomer is replaced by two Cu
þ
, with concomitant release of CopY from the
promoter and induction of transcription of the downstream genes. Copper is reduced to the cuprous form, Cu(I),
by an as-yet-uncharacterised reductase before being imported into the cell by CopA, whence it is carried in the
cytoplasm by the specific metallochaperone, CopZ. Copper is then transferred from CopZ to the dimeric
Zn-containing repressor CopY, displacing the Zn and releasing the CopY from the promoter. This allows tran-
scription of the four cop genes to proceed. Under high-copper conditions, excess CopZ is degraded by a copper-
activated protease.
