nan Richa

Molecular Mechanisms of Ultraviolet Radiation-Induced DNA Damage and Repair

DNA is one of the prime molecules, and its stability is of utmost importance for proper functioning and existence of all living systems. Genotoxic chemicals and radiations exert adverse effects on genome stability. Ultraviolet radiation (UVR) (mainly UV-B: 280–315 nm) is one of the powerful agents that can alter the normal state of life by inducing a variety of mutagenic and cytotoxic DNA lesions such as cyclobutane-pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers as well as DNA strand breaks by interfering the genome integrity. To counteract these lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). Additionally, double-strand break repair (by homologous recombination and nonhomologous end joining), SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms with the expense of specific gene products. This review deals with UV-induced alterations in DNA and its maintenance by various repair mechanisms.

Photoprotective compounds from marine organisms

The substantial loss in the stratospheric ozone layer and consequent increase in solar ultraviolet radiation on the earth’s surface have augmented the interest in searching for natural photoprotective compounds in organisms of marine as well as freshwater ecosystems. A number of photoprotective compounds such as mycosporine-like amino acids (MAAs), scytonemin, carotenoids and several other UV-absorbing substances of unknown chemical structure have been identified from different organisms. MAAs form the most common class of UV-absorbing compounds known to occur widely in various marine organisms; however, several compounds having UV-screening properties still need to be identified. The synthesis of scytonemin, a predominant UV-A-photoprotective pigment, is exclusively reported in cyanobacteria. Carotenoids are important components of the photosynthetic apparatus that serve both light-harvesting and photoprotective functions, either by direct quenching of the singlet oxygen or other toxic reactive oxygen species or by dissipating the excess energy in the photosynthetic apparatus. The production of photoprotective compounds is affected by several environmental factors such as different wavelengths of UVR, desiccation, nutrients, salt concentration, light as well as dark period, and still there is controversy about the biosynthesis of various photoprotective compounds. Recent studies have focused on marine organisms as a source of natural bioactive molecules having a photoprotective role, their biosynthesis and commercial application. However, there is a need for extensive work to explore the photoprotective role of various UV-absorbing compounds from marine habitats so that a range of biotechnological and pharmaceutical applications can be found.

Molecular characterization of hot spring cyanobacteria and evaluation of their photoprotective compounds

Phylogenetic analysis of 4 cyanobacterial strains isolated from hot springs in Rajgir, India, was carried out using the 16S rRNA gene (1400 bp). These strains were identified as members of Chroococcales ( Cyanothece sp. strain HKAR-1) and Nostocales ( Nostoc sp. strain HKAR-2, Scytonema sp. strain HKAR-3, and Rivularia sp. strain HKAR-4). Furthermore, we evaluated the presence of ultraviolet-screening and (or) photoprotective compounds, such as mycosporine-like amino acids (MAAs) and scytonemin, in these cyanobacteria by using high-performance liquid chromatography. Well-characterized MAAs, including the critical and highly polar compounds shinorine, porphyra-334, and mycosporine-glycine, as well as several unknown MAAs, were found in these hot-spring-inhabiting microorganisms. The presence of scytonemin was detected only in Scytonema sp. strain HKAR-3 and Rivularia sp. strain HKAR-4. The results indicate that hot spring cyanobacteria, namely Cyanothece, Nostoc, Scytonema, and Rivularia, belonging to different groups possess various photoprotective compounds to cope up with the negative impacts of damaging radiations.

Mycosporine-like amino acids profile and their activity under PAR and UVR in a hot-spring cyanobacterium Scytonema sp. HKAR-3

The cyanobacterium Scytonema sp. HKAR-3 isolated from a hot spring in India was investigated for the presence of mycosporine-like amino acids (MAAs) and their induction under PAR and PAR+UVR. High-performance liquid-chromatograph (HPLC) analysis revealed the presence of two MAAs, mycosporine–glycine (λmax = 310 nm) and an unknown MAA-334 (λmax = 334 nm), with retention times of 4.1 and 8.7 min, respectively. This is the first report for the presence of two MAAs and, in particular, the synthesis of mycosporine–glycine in any strain of Scytonema. There was no effect of radiation type (PAR or PAR+UVR) on the synthesis of mycosporine–glycine; however, the synthesis of MAA-334 was found to be higher in samples receiving PAR+UVR than in those receiving PAR only. There was a circadian induction in the synthesis of MAAs under alternate 12-h light (PAR or PAR+UVR) and dark periods. MAAs were found to be induced mostly during the light period and their production dropped again during the dark period. This suggests that the synthesis of MAAs is an energy-dependent process and depends on solar energy for its maintenance in natural habitats.

Physiological Aspects of UV-Excitation of DNA

Solar ultraviolet (UV) radiation, mainly UV-B (280-315 nm), is one of the most potent genotoxic agents that adversely affects living organisms by altering their genomic stability. DNA through its nucleobases has absorption maxima in the UV region and is therefore the main target of the deleterious radiation. The main biological relevance of UV radiation lies in the formation of several cytotoxic and mutagenic DNA lesions such as cyclobutane pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers (DEWs), as well as DNA strand breaks. However, to counteract these DNA lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, excision repair, and mismatch repair (MMR). Photoreactivation involving the enzyme photolyase is the most frequently used repair mechanism in a number of organisms. Excision repair can be classified as base excision repair (BER) and nucleotide excision repair (NER) involving a number of glycosylases and polymerases, respectively. In addition to this, double-strand break repair, SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms to ensure genomic stability. This review concentrates on the UV-induced DNA damage and the associated repair mechanisms as well as various damage detection methods.

Isolation and in silico analysis of Fe-superoxide dismutase in the cyanobacterium Nostoc commune.

Cyanobacteria are known to endure various stress conditions due to the inbuilt potential for oxidative stress alleviation owing to the presence of an array of antioxidants. The present study shows that Antarctic cyanobacterium Nostoc commune possesses two antioxidative enzymes viz., superoxide dismutase (SOD) and catalase that jointly cope with environmental stresses prevailing at its natural habitat. Native-PAGE analysis illustrates the presence of a single prominent isoform recognized as Fe-SOD and three distinct isoforms of catalase. The protein sequence of Fe-SOD in N. commune retrieved from NCBI protein sequence database was used for in silico analysis. 3D structure of N. commune was predicted by comparative modeling using MODELLER 9v11. Further, this model was validated for its quality by Ramachandran plot, ERRAT, Verify 3D and ProSA-web which revealed good structure quality of the model. Multiple sequence alignment showed high conservation in N and C-terminal domain regions along with all metal binding positions in Fe-SOD which were also found to be highly conserved in all 28 cyanobacterial species under study, including N. commune. In silico prediction of isoelectric point and molecular weight of Fe-SOD was found to be 5.48 and 22,342.98Da respectively. The phylogenetic tree revealed that among 28 cyanobacterial species, Fe-SOD in N. commune was the closest evolutionary homolog of Fe-SOD in Nostoc punctiforme as evident by strong bootstrap value. Thus, N. commune may serve as a good biological model for studies related to survival of life under extreme conditions prevailing at the Antarctic region. Moreover cyanobacteria may be exploited for biochemical and biotechnological applications of enzymatic antioxidants.

Effects of Global Change, Including UV and UV Screening Compounds

Primary productivity of phytoplankton and macroalgae depends on a number of external factors such as temperature, light availability and nutrients. It is affected by climate change which causes alterations in a number of parameters. Temperature increases result is changes in habitat selection and species composition and has a large impact on polar ecosystems by generating melt water which also affects species composition. Increasing solar UV radiation is a major stress factor for prokaryotic and eukaryotic phytoplankton. This is intensified by rising temperatures which result in higher stratification and a shallower mixing layer where the organisms are exposed to excessive visible and UV radiation. Phytoplankton and macroalgae counter this challenge by producing UV absorbing substances such as mycosporine-like amino acids and scytonemin-like, vertical migration and mat formation. Shoaling of the upper mixing layer also limits the accessibility to nutrients from deeper water layers. Increasing CO2 concentrations result in ocean acidification which affects productivity and calcification in animals, macroalgae and calcified phytoplankton.

Sensitivity of two Nostoc species harbouring diverse habitats to ultraviolet-B radiation

The effects of ultraviolet-B (UV-B; 280–315 nm) radiation on certain key physiological and biochemical processes were studied in two Nostoc species harbouring diverse habitats. Nostoc sp. strain HKAR-2, a thermophilic cyanobacterium, was isolated from the hot-spring, Rajgir, whereas, Nostoc sp. HKAR-6 was a rice-field isolate from Banaras Hindu University, Varanasi. Complete killing of the cells occurred after 48 h of UV-B exposure in Nostoc sp. strain HKAR-6, whereas in case of Nostoc sp. strain HKAR-2 death occurred only after 72 h. Chlorophyll a and phycocyanin content were found to be adversely affected by UV-B irradiation in both the test organisms. However, a progressive increase in carotenoids content was observed upto 10 h of UV-B exposure but subsequently declined after 12 h in both the Nostoc sp. The antioxidative enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and peroxidase that scavenge the UV-B generated harmful reactive oxygen species were found to have multifold induction in their content in both the strains following UV-B exposure in comparison to non-irradiated control cultures. In addition, these organisms also synthesize mycosporine-like amino acids (MAAs); able to carry out UV-screening. Porphyra-334 and shinorine were found to be the common MAAs in both the Nostoc sp., while an unknown MAAs with a retention time of 6.9 min (λmax—334 nm) was found in Nostoc sp. strain HKAR-2. Findings from this study suggest that both enzymatic and non-enzymatic defense mechanisms are being employed by Nostoc sp. strain HKAR-2 and Nostoc sp. strain HKAR-6 to counteract the damaging effects of UV-B radiation.

Nanobiotechnology of cyanobacterial UV-protective compounds: innovations and prospects

Abstract Cyanobacteria synthesize novel secondary UV-protective metabolites such as mycosporine-like amino acids (MAAs) and scytonemin. MAAs are composed of aminocyclohexenone or an aminocyclohexinimine chromophore conjugated with the nitrogen substituent of an amino acid or its amino alcohol and are small, water soluble, and colorless compounds. The extracellular polysaccharide sheath pigment scytonemin found in some cyanobacteria is a lipid-soluble dimeric pigment composed of indolic and phenolic subunits linked by an olefinic carbon atom. These compounds are highly photostable and act as potent photoprotectant and antioxidant. Inorganic nanoconjugates of these biomolecules could serve as a strong nontoxic, ecofriendly, biological sunscreen agent as they prolong their residence time in the target samples. Hence, future research must be focused on the investigation of facile, efficient, environmentally benign synthesis and use of these nanoparticle–biomolecule complexes in cosmetic and pharmaceutical industries as well as food industry to improve the shelf life of commodities.