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Fig. 6.6
Incorporation of
Mg
2+
into protoporphyrin
IX by Mg-chelatase
(Adapted from Kannangara
and von Wettstein
2010
)
membrane-bound enzyme increased upon addition of exogenous Mg (Lee
et al.
1992
). In pea chloroplasts, contrary to what was observed in cucumber
plastids, both stroma and membranes were needed to reconstitute Mg-Proto
chelatase activity (Walker and Weinstein
1991
). It is not known whether the
discrepancy between the cucumber and pea results is due to differences in prepara-
tory methodologies or not. Indeed it has been reported that the separation of plastid
stroma from plastid membranes may result in the solubilization of membrane
components if appropriate precautions are overlooked (Lee et al.
1991
). In
cucumber but not in pea, Mg-Proto chelatase was stabilized by its substrate
(i. e. exogenous Proto) before separation of the stroma from the plastid membranes
(Lee et al.
1992
).
6.2.3.2 Molecular Biology of Mg-Proto Chelatase
Mutational analysis of the
Rb
.
Capsulatus
photosynthesis gene cluster suggested
that three sequenced genes, namely
bchH
,
bchD
and
bchI
were involved in
Mg-chelation (Suzuki et al.
1997
). The
bchH
,
bchI
and
bchD
genes from
R
.
spheroides
were expressed in
E
.
coli
. When cell-free extracts from strains
containing the gene products
BchH
,
BchI
, and
BchD
were combined, the mixture
was able to catalyze the insertion of Mg into Proto in an ATP-dependent manner
(Gibson et al.
1995
). The authors suggested that
BchH
binds Proto prior to the
insertion of the Mg atom. Also genes from
Synechocystis
PCC6803 a cyanobacte-
rium, with homology to the
bchH
,
bchD
and
bchI
genes, namely
chlH
,
chlD
and
chlI
, were cloned and overexpressed in
E
.
coli
(Jensen et al.
1995
). In this case too,
the combined cell-free extracts containing the
ChlH
,
ChlI
and
ChlD
gene products
were able to catalyze the insertion of Mg
2+
into Proto in an ATP-dependent manner.
The N-terminal half of the ChlD protein exhibited a 40-41 % homology to
Rhodobacter BchI
and
Synechocystis ChlI
, whereas the C-terminal half displayed
a 33 % homology to
Rhodobacter BchD
. The authors suggested the existence of an
evolutionary relationship between the
I
and
D
genes.
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