632
Chlamydomonas
Light
Carbohydrates
NADP
NADPH
ADP
ADP
CO
2
fixation
CO
2
FNR
Fd
PQ
PC
Antenna
Chl
e
Antenna
Chl
e
PQH
2
H
+
H
+
H
+
H
2
O½O
2
+
2H
+
PSII
cytb6/f
PSI
ATP synthase
Lumen
Stroma
Thylakoid
membrane
Fig. 5
Scheme of photosynthetic electron transport chain showing the four
complexes – photosystem II (PSII) and photosystem I (PSI) with their associated chlorophyll
antennae, the cytochrome
b
6
/
f
complex (cytb6/f), and the ATP synthetase. Fd, ferredoxin; FNR,
ferredoxin-NADP reductase; PQ, plastoquinone; PQH
2
, plastoquinol; chl, chlorophyll.
the stroma or on the thylakoid membranes
to form functional protein complexes.
Because photosynthetic function is dis-
pensable when
C. reinhardtii
cells are
grown on a medium containing acetate,
it has been possible to isolate numerous
nuclear and chloroplast mutants deFcient
in photosynthetic activity. An important
feature of the thylakoid membrane is
that the physical–chemical properties of
its numerous pigments depend critically
on the functional state of photosynthetic
complexes. ±luorescence has, therefore,
been used as a powerful, noninvasive
method for screening photosynthetic mu-
tants. Mutations affecting photosynthesis
fall into two major classes. The Frst in-
cludes mutations within genes encoding
components of the photosynthetic sys-
tem. The second class includes mostly
nuclear mutations that act indirectly on
photosynthesis. Many of these mutations
affect genes whose products are required
for the proper expression of chloroplast
genes. These factors appear to act at several
posttranscriptional levels (e.g. chloroplast
RNA stability, RNA processing and splic-
ing, translation) and, most probably, at
the level of assembly of the photosynthetic
complexes. Surprisingly, the number of
the nuclear loci involved in chloroplast
gene expression is quite large and most
of their products appear to act in a gene-
speciFc manner (±ig. 6).
4.1.1
Mutations Affecting Structural
Photosynthetic Genes
Analysis of photosynthetic mutants has
revealed that several are affected in the
structural genes of the photosynthetic
complexes. Characterized chloroplast mu-
tations comprise deletions, sequence du-
plications, and single–base pair changes.
Among the latter, herbicide-resistant mu-
tants have been especially useful for
probing the structure–function relation-
ship of the D1 subunit of photosystem
II. Earlier studies had revealed that the
absence of any of the subunits from the
core complex of photosystem II or I usu-
ally leads to a drastic destabilization of
the other subunits of the complex. It
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