Chlamydomonas
637
a consequence of loss of plastocyanin
or decreased photosynthetic activity in
copper-deFcient cells.
4.1.4
Adaptation of the Photosynthetic
Apparatus to Changes in Light Conditions
Like other algae and land plants,
C. rein-
hardtii
has the remarkable ability to adapt
its photosynthetic machinery to changes in
light quantity and quality. Photosynthetic
organisms can dissipate the light excitation
energy by photochemistry, by fluores-
cence, or by nonphotochemical quench-
ing. The latter occurs under excessive
illumination, is triggered by the increased
proton gradient across the thylakoid mem-
brane, and involves a reversible structural
modiFcation of the carotenoids through
the xanthophyll cycle. This leads to an
increased heat dissipation of the excita-
tion energy and lowers the fluorescence
emission and the photooxidative damage
within the photosynthetic reaction cen-
ters. A genetic approach has identiFed
several factors involved in nonphotochem-
ical quenching and has provided direct
evidence for the involvement of the xan-
thophylls in this process.
Photosystem II and photosystem I act
in series in the photosynthetic electron
transport chain, and they are connected
to two distinct antennae systems with
different light absorption properties. Upon
a change in the spectral quality of the
exciting light, a reorganization of the
antennae occurs, insuring a balanced
excitation of the two photosystems and
hence an optimal photosynthetic quantum
yield. This process is called
state transition
and involves the displacement of the
antenna of photosystem II to photosystem
I under conditions in which photosystem
II is overstimulated relative to photosystem
I. A key step of this mechanism is the
activation of a kinase that speciFcally
phosphorylates the N-terminal end of
the LHCII proteins. The activation is
triggered through a signal transduction
chain that involves the redox state of
the plastoquinone pool and a functional
cytochrome
b
6
f
complex. Attempts to
isolate the kinase by biochemical means
have failed. However, a genetic approach
has identiFed several mutants that are
deFcient in state transition and blocked in
the phosphorylation of LHCII. Recently,
the gene deFcient in one of these mutants
has been isolated and characterized and
has been found to encode a thylakoid-
associated kinase. Mutants of this sort offer
promising
possibilities
for
identifying
the different factors involved in state
transition.
4.1.5
Heteroplasmicity of the Chloroplast
Genome
It is generally assumed that chloroplast
genomes consist of identical copies of
single DNA molecules. The predomi-
nantly uniparental inheritance of chloro-
plast genomes could easily maintain such
homoplasmicity. Although most hetero-
plasmic markers segregate rapidly to form
homoplasmic cells, cases of stable hetero-
plasmicity can occur in the chloroplast of
C. reinhardtii
.
Mutants deFcient in photosynthetic ac-
tivity have been shown to arise from
nonsense mutations within the chloro-
plast
rbcL
gene encoding the large subunit
of ribulose 1,5-bisphosphorate carboxy-
lase/oxygenase. In these mutants, the
holoenzyme is undetectable because of
rapid degradation of the unassembled
and truncated subunits. Photosynthetically
competent suppressors of these nonsense
mutants have been found to be hetero-
plasmic, giving rise to both mutant and
suppressor cells during divisions under
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