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 128

To study the role of DNA damage in the protein-synthesis-dependent repair of PSII,
the photolyase deficient cell cultures, which were grown under visible light, were exposed
to UV-B radiation. When protein synthesis was blocked by lincomycin, the oxygen
evolution activity was inhibited with the same rate in the mutant and the wild-type cells. In
the absence of lincomycin, the UV-B-induced loss of oxygen evolution was not
significantly different in the mutant and wild-type cells up to one hour, but after longer
illumination the activity of the mutant was lost somewhat faster than that of the wild-type
cells (Figure 10). According to these data the lack of photolyase activity does not influence
the damage of the PSII complex by UV-B radiation. However, the efficiency of protein
repair is somewhat slowed down after relatively short (few hours) exposure times, showing
that unrepaired DNA accumulates and retards de novo synthesis of the D1 and D2 reaction
center subunits required for the restoration of PSII activity.

5. Damage of phycobilisomes by UV-B

UV-B radiation affects not only the electron transport processes mediated by the
PSII complex, but also the light harvesting antenna systems. In cyanobacteria light capture
is performed mainly by phycobiliproteins which form large rod-like structures, the so called
phycobilisomes. Prolonged exposure to UV-B radiation damages the phycobilisomes,
leading to the loss of their characteristic absorption at 600-650 nm (Figure 11) and inhibits
the transfer of energy to the photosynthetic reaction centers 8,36-37 . However, the
UV-sensitivity of phycobilisomes relative to PSII has not been studied previously. In order

Control 2 h UV-B

1 h UV-B

0.3

3 h UV-B

0.2

3 h UV-B +
3 h recovery

0.1

0.0

400 500 600 700 800

Wavelength (nm)

Figure 11. The effect of UV-B radiation on the absorption of phycobilisomes. Synechocystis 6803 cell
were irradiated with 6 PEm -2 s -1 UV-B complemented with 250 PEm -2 s -1 visible light and the absorption
spectrum of the cell suspension was measured after the indicated exposure times. After the UV-B
treatment the samples were transferred to visible light for a 3 h recovery period.