UV-B AND UV-A RADIATION EFFECTS ON PHOTOSYNTHESIS AT THE MOLECULAR LEVEL - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 124

4. The repair of UV-induced damage of PSII

Repair via de novo protein synthesis

One of the main defense responses of intact photosynthetic cells against UV-B
damage is the repair of PSII via de novo synthesis of the D1 and D2 proteins 26 (Figure. 7).

D2

D1

D2

D1

Q A Q B

Fe

UV damage

Phe Phe

P 680
Tyr-Z

Tyr-D

Mn

Religation
Activation

Protein
degradation

D1

D2

D2

D1

Reassembly De novo

protein
synthesis

D2

D1

Figure 7. The scheme of PSII repair . UV radiation induces the damage and degradation of the D1
and D2 reaction center subunits. The first step of the repair is the de novo synthesis of D1 and D2.
This is followed by reassembly of the reaction center, and the religation and activation of the redox
cofactors.

We have recently shown that in the cyanobacterium Synechocystis 6803 UV-B light
has differential effect on the transcription of the psbA2 and psbA3 genes encoding identical
D1 proteins. UV-B irradiation induces a large increase in the accumulation of the psbA3
mRNA, which gives only a minor fraction (<10 %) of the psbA mRNA pool under normal
conditions in wild-type cells 27 . In contrast, only a small increase was observed in the level
of the normally highly expressed psbA2 mRNA. Similar differential response of the psbD1
and psbD2 genes, encoding identical D2 proteins, was also observed in response to UV-B
light 34 . These results show that psbA3 and psbD2 act as UV-B stress genes which are
needed for rapid repair of the damaged PSII reaction center subunits.