12
A. Trebst
light. This is particularly true when there is no electron acceptor present. In
vivo the terminal electron acceptor of photosynthesis is, of course, CO 2 , The
bleaching of the photo systems in vivo in the absence of CO2 is the longknown physiological phenomenon of photoinhibition. (For a recent review
see Demmig-Adams and Adams 1992). We are beginning to understand
the molecular . mechanism of photoinhibition (for review see Barber and
Andersson 1992; Prasil et al. 1992). It becomes apparent that the inactivation
of photo system II is due to damage by a modification of an amino acid in the
D1 protein. This inactivation of photo system II can be repaired, but an
exchange of an amino acid in a peptide sequence is not possible. For the
repair, the damaged D1 protein has to be fully degraded first and then newly
synthesized and reassembled back to functionally active photosystem II.
This damage and repair cycle is a costly process. It is assumed that photoinhibition will occur at any light intensity in the field (Greer et al. 1991). It is
estimated that it might reduce the efficiency of photosynthesis by up to 15%.
Under low light intensities, degradation is masked in vivo by the repair
process. A continuous degradation and resynthesis of the D1 protein is
called the "rapid turnover" of the D1 protein (Mattoo et al. 1984, 1989).
Only when the light-dependent destruction of the D1 protein can no longer
be compensated for by the repair process, does the inactivation of photosynthetic electron flow become apparent as loss of the photosynthesis
capacity. This has been intensively studied in Chlamydomonas rho (Schuster
et al. 1988, Prasil et al. 1992).
Rapid turnover has been discovered by radioactive pulse chase experiments (Mattoo et al. 1984). Of the many protein subunits of PS II
discussed above, only the D1 protein is labeled quickly in a light pulse - in a
matter of minutes in higher plants - with a suitable radioactive precursor
like radioactive sulfate or an amino acid. The label also disappears quickly
by a chase in the light, indicating the degradation of the protein.
The mechanism of photo degradation of the D1 protein is not fully
understood in all aspects. Probably in the absence of an electron acceptor
that draws the electron from the plastohydroquinone pool, the QB site
plastoquinone remains reduced or leaves the site empty. Then QA also
becomes fully reduced, possibly even to the double reduced form, and also
may leave its site (Barber and Andersson 1992; Vass et al. 1992). Then the
electron from the charge separation of the reaction center finds no quinone
to reduce. On recombination, the triplet of the P680 is formed and reacts
with oxygen to form the highly toxic singlet oxygen. It can be quenched to a
certain extent by a carotene. The singlet oxygen may modify amino acid
residues in the D1 protein, rendering the protein inactive. The modified D1
protein has to be removed completely and replaced by a new copy.
The primary cleavage site in the D1 protein during photoinhibition is a
matter of debate at present (Barber and Andersson 1992). In low light rapid
turnover, the actual degradation of the D 1 protein begins at a cleavage site
in the amino acid sequence assumed to be in the extended loop (Mattoo et
A. Trebst
light. This is particularly true when there is no electron acceptor present. In
vivo the terminal electron acceptor of photosynthesis is, of course, CO 2 , The
bleaching of the photo systems in vivo in the absence of CO2 is the longknown physiological phenomenon of photoinhibition. (For a recent review
see Demmig-Adams and Adams 1992). We are beginning to understand
the molecular . mechanism of photoinhibition (for review see Barber and
Andersson 1992; Prasil et al. 1992). It becomes apparent that the inactivation
of photo system II is due to damage by a modification of an amino acid in the
D1 protein. This inactivation of photo system II can be repaired, but an
exchange of an amino acid in a peptide sequence is not possible. For the
repair, the damaged D1 protein has to be fully degraded first and then newly
synthesized and reassembled back to functionally active photosystem II.
This damage and repair cycle is a costly process. It is assumed that photoinhibition will occur at any light intensity in the field (Greer et al. 1991). It is
estimated that it might reduce the efficiency of photosynthesis by up to 15%.
Under low light intensities, degradation is masked in vivo by the repair
process. A continuous degradation and resynthesis of the D1 protein is
called the "rapid turnover" of the D1 protein (Mattoo et al. 1984, 1989).
Only when the light-dependent destruction of the D1 protein can no longer
be compensated for by the repair process, does the inactivation of photosynthetic electron flow become apparent as loss of the photosynthesis
capacity. This has been intensively studied in Chlamydomonas rho (Schuster
et al. 1988, Prasil et al. 1992).
Rapid turnover has been discovered by radioactive pulse chase experiments (Mattoo et al. 1984). Of the many protein subunits of PS II
discussed above, only the D1 protein is labeled quickly in a light pulse - in a
matter of minutes in higher plants - with a suitable radioactive precursor
like radioactive sulfate or an amino acid. The label also disappears quickly
by a chase in the light, indicating the degradation of the protein.
The mechanism of photo degradation of the D1 protein is not fully
understood in all aspects. Probably in the absence of an electron acceptor
that draws the electron from the plastohydroquinone pool, the QB site
plastoquinone remains reduced or leaves the site empty. Then QA also
becomes fully reduced, possibly even to the double reduced form, and also
may leave its site (Barber and Andersson 1992; Vass et al. 1992). Then the
electron from the charge separation of the reaction center finds no quinone
to reduce. On recombination, the triplet of the P680 is formed and reacts
with oxygen to form the highly toxic singlet oxygen. It can be quenched to a
certain extent by a carotene. The singlet oxygen may modify amino acid
residues in the D1 protein, rendering the protein inactive. The modified D1
protein has to be removed completely and replaced by a new copy.
The primary cleavage site in the D1 protein during photoinhibition is a
matter of debate at present (Barber and Andersson 1992). In low light rapid
turnover, the actual degradation of the D 1 protein begins at a cleavage site
in the amino acid sequence assumed to be in the extended loop (Mattoo et