Rajeshwar P. Sinha

UV-induced DNA damage and repair: a review

Increases in ultraviolet radiation at the Earths surface due to the depletion of the stratospheric ozone layer have recently fuelled interest in the mechanisms of various effects it might have on organisms. DNA is certainly one of the key targets for UV-induced damage in a variety of organisms ranging from bacteria to humans. UV radiation induces two of the most abundant mutagenic and cytotoxic DNA lesions such as cyclobutane-pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs) and their Dewar valence Isomers. However, cells have developed a number of repair or tolerance mechanism to counteract the DNA damage caused by UV or any other stressors. Photoreactivation with the help of the enzyme photolyase is one of the most important and frequently occurring repair mechanisms in a variety of organisms. Excision repair, which can be distinguished into base excision repair (BER) and nucleotide excision repair (NER), also plays an important role in DNA repair in several organisms with the help of a number of glycosylases and polymerases, respectively. In addition, mechanisms such as mutagenic repair or dimer bypass, recombinational repair, cell-cycle checkpoints, apoptosis and certain alternative repair pathways are also operative in various organisms. This review deals with UV-induced DNA damage and the associated repair mechanisms as well as methods of detecting DNA damage and its future perspectives.

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.

Effects of UV irradiation on certain physiological and biochemical processes in cyanobacteria

The effects of artificial UV (280–400 nm, 5 W m−2) radiation on heterocyst differentiation, nitrogenase activity, 14CO2 uptake and protein profile of whole cell and isolated heterocysts have been studied in four cyanobacterial strains isolated from Indian rice paddy fields. Exposure of cells to UV for 1 h significantly affected the differentiation of vegetative cells into heterocysts in four cyanobacterial strains studied (Anabaena sp., Nostoc sp., Nostoc carmium and Scytonema sp). Almost 50% fewer heterocysts were recorded in Anabaena sp. and Scytonema sp. and nearly 70% fewer in Nostoc sp. and Nostoc carmium after UV radiation in comparison with controls without UV. Nitrogenase activity in Anabaena sp. was completely inhibited within 45 min of UV exposure. 14CO2 uptake in Anabaena sp. was also severely affected by UV radiation. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS PAGE) analyses of the total protein profile of Anabaena sp. showed a linear decrease in the protein content with increasing UV exposure time. Almost complete elimination of most of the protein bands occurred after 120 min of UV exposure. The SDS PAGE protein profile of isolated heterocysts of Anabaena sp. showed three prominent polypeptides of 26, 54 and 55 kDa, with a decrease in the first two and complete elimination of the last one after 1 h of UV radiation.

Cyanobacteria and ultraviolet radiation (UVR) stress: Mitigation strategies

Cyanobacteria are primitive photosynthetic oxygen-evolving prokaryotes that appeared on the Earth when there was no ozone layer to protect them from damaging ultraviolet radiation (UVR). UVR has both direct and indirect effects on the cyanobacteria due to absorption by biomolecules and UVR-induced oxidative stress, respectively. However, these organisms have developed several lines of mitigation strategies/defense mechanisms such as avoidance, scavenging, screening, repair and programmed cell death to counteract the damaging effects of UVR. This review presents an update on the effects of UVR on cyanobacteria and the defense mechanisms employed by these prokaryotes to withstand UVR stress. In addition, recent developments in the field of molecular biology of UV-absorbing compounds such as mycosporine-like amino acids and scytonemin, are also added and the possible role of programmed cell death, signal perception as well their transduction under UVR stress is being discussed.

Photobiology and Ecophysiology of Rice Field Cyanobacteria

Cyanobacteria (blue-green algae) are phylogenetically a primitive group of gram-negative prokaryotes being the only bacteria to possess higher plant-type oxygenic photosynthesis. They also have the capacity to metabolize CO,, O,, N2 and H2 (1). Fossil evidence dates their appearance to the Precambrian era. Stromatolithic mats composed of cyanobacteria are dated at over 2.8 (or possibly 3.5) X lo9 years and appear nearly identical to those found today (2,3). Cyanobacteria are the largest and most widely distributed group of photosynthetic prokaryotes on earth and as a group are thought to have survived from a wide spectrum of environmental stresses such as heat and cold shock, drought, salinity, nitrogen starvation, photooxidation, anaerobiosis, osmotic and UV stress, erc. (4-6). Cyanobacteria are unique in having a cosmopolitan distribution ranging from hot springs to arctic regions. Cyanobacterial colonization of oceans, lakes, rivers, hot springs and soils, and their presence as symbiotic organisms in fungi and plants demand high variability in adapting to diverse environmental factors. Cyanobacterial populations occupy an important place in both aquatic as well as terrestrial ecosystems and achieve net photosynthesis even at temperatures as low as -7°C (7) and as high as 75°C (8). Members of cyanobacteria also possess a central position in the nutrient cycling largely due to their inherent capacity to fix atmospheric N2 with the help of the enzyme nitrogenase, directly into ammonium (MI4+). a form through which nitrogen enters into the food chain (9). Estimates vary as to the contribution of biological nitrogen fixation (BNF)? to the global nitrogen cycle, but according to one assumption cyanobacteria fix over 35 million tons of nitrogen annually, which is thus available for use by higher plants (10). In several instances the availability of

The cyanotoxin-microcystins: current overview

The monocyclic heptapeptides microcystins (MCs), are a group of hepatotoxins, produced worldwide by some bloom-forming cyanobacterial species/strains both in marine and freshwater ecosystems. MCs are synthesized non-ribosomally by large multi-enzyme complexes consisting of different modules including polyketide synthases and non-ribosomal peptide synthetases, as well as several tailoring enzymes. More than 85 different variants of MCs have been reported to exist in nature. These are chemically stable, but undergo bio-degradation in natural water reservoirs. Direct or indirect intake of MCs through the food web is assumed to be a highly exposed route in risk assessment of cyanotoxins. MCs are the most commonly found cyanobacterial toxins that cause a major challenge for the production of safe drinking water and pose a serious threat to global public health as well as fundamental ecological processes due to their potential carcinogenicity. Here, we emphasize recent updates on different modes of action of their possible carcinogenicity. Besides the harmful effects on human and animals, MC producing cyanobacteria can also present a harmful effect on growth and development of agriculturally important plants. Overall, this review emphasizes the current understanding of MCs with their occurrence, geographical distribution, accumulation in the aquatic as well as terrestrial ecosystems, biosynthesis, climate-driven changes in their synthesis, stability and current aspects on its degradation, analysis, mode of action and their ecotoxicological effects.

Induction of a mycosporine-like amino acid (MAA) in the rice-field cyanobacterium Anabaena sp. by UV irradiation

Abstract An ultraviolet-absorbing mycosporine-like amino acid (MAA) has been detected in a filamentous and heterocystous cyanobacterium, Anabaena sp., isolated from a rice paddy field near Varanasi, India. The MAA is isolated and purified by high-performance liquid chromatography. Only one MAA is detected with a retention time of around 2.8 min and an absorption maximum at 334 nm. The MAA is identified as shinorine, a bisubstituted MAA containing both glycine and serine groups. There is an increase in the amount of MAA when the cultures are exposed to photosynthetically active radiation and ultraviolet radiation (PAR+UV) in comparison with the cultures exposed to PAR only. This shows that UV stress induces the synthesis of MAA in this cyanobacterium and thus may protect the organism against deleterious high solar radiation, particularly during summer seasons in the tropics.

Mycosporine-like amino acids in the marine red alga Gracilaria cornea — effects of UV and heat

Ultraviolet (UV)-absorbing mycosporine-like amino acids (MAAs) were separated from a marine red alga Gracilaria cornea using HPLC. The isolated MAAs were identified as porphyra-334 and:or shinorine by comparing them with various standards. No in vivo induction of MAAs was detected in G. cornea even when the organism was grown for 4‐5 days either in the presence of UV-A and UV-B only or in combination with photosynthetically active radiation. In vitro absorption properties of MAAs were unaffected when irradiated with UV-B or subjected to heat treatment (7592°C) for up to 6 h. In comparison to MAAs, other pigments such as chlorophyll a (436 and 665 nm), carotenoids (475 nm) and phycocyanin (618 nm) were severely affected by UV-B irradiation. The results indicate a highly stable nature of MAAs against the environmental stress factors like UV-B and heat. The SDS-PAGE protein profile of G. cornea showed a gradual decrease in the intensity of protein bands at 20 kDa (a and b subunits of phycocyanin) but at the same time a gradual increase in the intensity of protein bands at 26 kDa (phycoerythrin), showing the phenomenon of chromatic adaptation (changes in pigmentation in photosynthetic organisms in response to light quality), when the organism was grown in the presence of UV plus PAR.

Genome mining of mycosporine-like amino acid (MAA) synthesizing and non-synthesizing cyanobacteria: A bioinformatics study

Mycosporine-like amino acids (MAAs) are a family of more than 20 compounds having absorption maxima between 310 and 362 nm. These compounds are well known for their UV-absorbing/screening role in various organisms and seem to have evolutionary significance. In the present investigation we tested four cyanobacteria, e.g., Anabaena variabilis PCC 7937, Anabaena sp. PCC 7120, Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 6301, for their ability to synthesize MAA and conducted genomic and phylogenetic analysis to identify the possible set of genes that might be involved in the biosynthesis of these compounds. Out of the four investigated species, only A. variabilis PCC 7937 was able to synthesize MAA. Genome mining identified a combination of genes, YP_324358 (predicted DHQ synthase) and YP_324357 (O-methyltransferase), which were present only in A. variabilis PCC 7937 and missing in the other studied cyanobacteria. Phylogenetic analysis revealed that these two genes are transferred from a cyanobacterial donor to dinoflagellates and finally to metazoa by a lateral gene transfer event. All other cyanobacteria, which have these two genes, also had another copy of the DHQ synthase gene. The predicted protein structure for YP_324358 also suggested that this product is different from the chemically characterized DHQ synthase of Aspergillus nidulans contrary to the YP_324879, which was predicted to be similar to the DHQ synthase. The present study provides a first insight into the genes of cyanobacteria involved in MAA biosynthesis and thus widens the field of research for molecular, bioinformatics and phylogenetic analysis of these evolutionary and industrially important compounds. Based on the results we propose that YP_324358 and YP_324357 gene products are involved in the biosynthesis of the common core (deoxygadusol) of all MAAs.