14
A. Trebst
between degradation and repair can come about suddenly just by a further
small increase in the level of stress. Photosystem II can tolerate an increase
in its inactivation to quite some extent; but it can do so only up to the
capacity of its repair system.
1.6 Regulation of Photosystem II by Phosphorylation
A number of proteins of the thylakoid membrane are phosphorylated usually
at a threonine at or close to the N-terminus see (Allen 1992).
Phosphorylation is a positioning of a strong charge on the protein that
may be essential for its proper folding and insertion into the membrane. The
phosphorylation of the 25 kDa light-harvesting protein results in a conformational change that in turn disconnects the complex from the PS II core
antenna system. In this way phosphorylation controls the light distribution
of excitation energy between the two photosystems, and at the same time
also the extent of grana stacking (Allen 1992). The significance of phosphorylation for the other membrane proteins is less clear. Among the core
photosystem II proteins, the 9 kDa product of the psbH gene appears to be
always phosphorylated. The phosphorylation of the 43 kDq antenna subunit
and of the D1 and D2 proteins appears to be intermittent. It appears that a
phosphorylated D1 protein is functionally not different from the unphosphorylated form, but it may play an important role in photo inhibition by
stabilizing the D1 protein (Aro et al. 1992; Elich et al. 1992).
The kinase that is phosphorylating the light-harvesting protein has been
purified. It is very interesting that it is a redox-controlled kinase by its
attachement to the cytochrome b6/f complex (Gal et al. 1990), most likely by
a quinone-binding site on the kinase. This might be true also for the control
of the still unknown protease in the rapid turnover of the Dl protein
(Bracht and Trebst, in preparation).
The redox control of the phosphorylating/dephosphorylating system may
open a new understanding how the capacity and actual efficiency of the
electron flow system in photosynthesis is regulated and adjusted to the need
of the assimilatory process coupled to it.
1. 7 Conclusions
Site-specific mutations in the psbA gene greatly help in modeling the threedimensional folding of the Dl reaction center polypeptide of photosystem
II. The phenomenon of rapid turnover of the D1 protein followed by
its cleavage, and then the complete degradation of D1 protein and of photoinhibition of photosystem II is discussed on the molecular and structural
A. Trebst
between degradation and repair can come about suddenly just by a further
small increase in the level of stress. Photosystem II can tolerate an increase
in its inactivation to quite some extent; but it can do so only up to the
capacity of its repair system.
1.6 Regulation of Photosystem II by Phosphorylation
A number of proteins of the thylakoid membrane are phosphorylated usually
at a threonine at or close to the N-terminus see (Allen 1992).
Phosphorylation is a positioning of a strong charge on the protein that
may be essential for its proper folding and insertion into the membrane. The
phosphorylation of the 25 kDa light-harvesting protein results in a conformational change that in turn disconnects the complex from the PS II core
antenna system. In this way phosphorylation controls the light distribution
of excitation energy between the two photosystems, and at the same time
also the extent of grana stacking (Allen 1992). The significance of phosphorylation for the other membrane proteins is less clear. Among the core
photosystem II proteins, the 9 kDa product of the psbH gene appears to be
always phosphorylated. The phosphorylation of the 43 kDq antenna subunit
and of the D1 and D2 proteins appears to be intermittent. It appears that a
phosphorylated D1 protein is functionally not different from the unphosphorylated form, but it may play an important role in photo inhibition by
stabilizing the D1 protein (Aro et al. 1992; Elich et al. 1992).
The kinase that is phosphorylating the light-harvesting protein has been
purified. It is very interesting that it is a redox-controlled kinase by its
attachement to the cytochrome b6/f complex (Gal et al. 1990), most likely by
a quinone-binding site on the kinase. This might be true also for the control
of the still unknown protease in the rapid turnover of the Dl protein
(Bracht and Trebst, in preparation).
The redox control of the phosphorylating/dephosphorylating system may
open a new understanding how the capacity and actual efficiency of the
electron flow system in photosynthesis is regulated and adjusted to the need
of the assimilatory process coupled to it.
1. 7 Conclusions
Site-specific mutations in the psbA gene greatly help in modeling the threedimensional folding of the Dl reaction center polypeptide of photosystem
II. The phenomenon of rapid turnover of the D1 protein followed by
its cleavage, and then the complete degradation of D1 protein and of photoinhibition of photosystem II is discussed on the molecular and structural