IMPACT OF UV RADIATION ON RICE-FIELD CYANOBACTERIA: ROLE OF PHOTOPROTECTIVE COMPOUNDS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 213

Effects on photosynthetic enzymes
The activity of the ribulose 1,5-bisphosphate carboxylase (RuBISCO), the
primary CO 2 fixing enzyme, has been reported to be inhibited by UV-B irradiation in a
number of cyanobacteria which may be due to protein destruction or enzyme
inactivation 32 . The control of RuBISCO biosynthesis is strongly influenced by the
prevailing light environment. During UV-B irradiation, proteins may undergo a number
of modifications including photodegradation, increased aqueous solubility of membrane
proteins, and fragmentation of the peptide chain, leading to inactivation of proteins
(enzymes) and disruption of their structural entities 32,39 . UV-induced inhibition of 14 CO 2
uptake in various rice-field cyanobacteria has been reported which could be due to the
effect on the photosynthetic apparatus leading to the reduction in the supply of ATP and
NADPH 2 41-42 . A disruption of the cell membrane and/or alteration in thylakoid integrity
as a result of UV-B irradiation may partly or wholly destroy the components required
for photosynthesis and may thus affect the rate of CO 2 fixation 32,41 . UV-induced loss of
photosystem II activity 44 and opening of the membrane-bound calcium channels have
been demonstrated in cyanobacteria 45 .
Effects on DNA
The peak absorption of DNA lies in the UV-C range that is absorbed in the upper
layers of the atmosphere and does not reach the Earth's surface. However, the
absorption of UV-B radiation by DNA is sufficient to induce severe damage to the DNA
in cyanobacteria. Absorbed quanta of UV can induce changes in the molecular structure
of the DNA 46 . The two major classes of mutagenic DNA lesions induced by UV
radiation are cis-syn cyclobutane-pyrimidine dimers (CPDs) and pyrimidine (6-4)
pyrimidone photoproducts (6-4PPs). 6-4PPs are formed at 20 - 30 % of the yields of
CPDs 18,47 . Both classes of lesions distort the DNA helix. CPDs and 6-4PPs induce a
bend or kink of 7 - 9° and 44°, respectively. The ability of UV radiation to damage a
given base is determined by the flexibility of the DNA. Sequences that facilitate
bending and unwinding are favourable sites for damage formation, e.g. CPDs are
formed at higher yields in single-stranded DNA, at the flexible ends of poly(dA)(dT)
tracts, but not in their rigid centre. Bending of DNA towards the minor groove reduces
CPD formation. One of the transcription factors having a direct effect on DNA damage
formation and repair is the TATA-box binding protein (TBP). TBP promotes the
selective formation of 6-4PPs in the TATA-box, where the DNA is bent, but CPDs are
formed at the edge of the TATA-box and outside, where DNA is not bent. These DNA
lesions interfere with DNA transcription and replication and can lead to misreadings of
the genetic code and cause mutations and death 17-18,47 . UV-induced thymine dimer and
other photoproducts formation have been reported in a number of cyanobacteria 48-50 .
4. Impacts of UV-B radiation on cyanobacteria in polar ecosystems
In addition to the tropics, cyanobacteria are also common organisms in the
freshwater and terrestrial ecosystems of Antarctica and also in the north polar regions 51 .
Cyanobacteria have colonized a wide variety of polar environments including soils, the