Ultraviolet-B photodestruction of a light-harvesting complex.

Abstract

Cyanobacteria are important contributors to global photosynthesis in both marine and terrestrial environments. Quantitative data are presented on UV-B-induced damage to the major cyanobacterial photosynthetic light harvesting complex, the phycobilisome, and to each of its constituent phycobiliproteins. The photodestruction quantum yield, 4¢295 nm. for the phycobiliproteins is high (_10-3, as compared with _10-7 for visible light). Energy transfer on a picosecond time scale does not compete with photodestruction. Photodamage to phycobilisomes in vitro and in living cells is amplified by causing dissociation and loss of function of the complex. In photosynthetic organisms, UV-B damage to lightharvesting complexes may significantly exceed that to DNA.In cells of nonphotosynthetic organisms, DNA absorbs '50% of the incident UV-B radiation and is the primary target of photodamage. However, in plants and photosynthetic organisms, chlorophyll and other pigments may contribute significantly to shielding of the DNA from ultraviolet radiation. For example, comparison of absolute action spectra for pyrimidine dimer formation in alfalfa seedlings versus T7 bacteriophage DNA show over a 100-fold decrease in the rate of UV-Binduced DNA damage (1). In cyanobacteria, the most widespread and abundant photosynthetic prokaryotes, we calculate that the light-harvesting proteins (phycobiliproteins and chlorophyll proteins) account for >99% of the UV-B absorption. There are voluminous data on the photochemistry of DNA and on quantum yields of its various photoreactions (e.g., ref. 2). Comparable data on the photodestruction quantum yields of proteins in general and light-harvesting proteins in particular are totally lacking.To address this deficiency, we present a quantitative study of the photodestruction of the cyanobacterial light-harvesting complex, the phycobilisome (3), by ultraviolet light of 285-305 nm at a photon flux for this 20-nm wavelength band about 50 times that reaching the Earth's surface at the normal stratospheric ozone level of about 300 Dobson units [matm-cm (1 atm = 101.33 kPa)] (4). Cyanobacteria make a dominant contribution to phytoplankton primary productivity. In the range from 285 to 700 nm, phycobilisomes account for over half of the absorption cross-section of cyanobacterial cells.Chlorophyll-protein complexes account for most of the remaining absorbance.As a test organism, we chose the extensively studied filamentous heterocystous cyanobacterium Anabaena strain PCC 7120. A schematic representation of the Anabaena phycobilisome is shown in Fig. 1. Allophycocyanin (AP) is the major component of the phycobilisome core (proximal to the reaction centers within the thylakoid membrane), and C-phycocyanin (PC), and phycoerythrocyanin (PEC) are the major and minor components of the rod substructures, respectively. We have studied the photochemistry of the individual phycobiliproteins, of the purified intact as well as dissociated phycobilisomes, and of phycobilisomes in living Anabaena cells. Spectroscop...

Connecting thermal physiology and latitudinal niche partitioning in marine Synechococcus

“…M16.1, for which fast specific degradation of the distal PBS hexamers (PEII) was evidenced by MpeB immunoblotting (Figure 4a). Our results are comparable to previous studies led in other cyanobacteria that pointed out serious PBS perturbations in response to ultraviolet (Lao and Glazer, 1996;Rajagopal and Murthy, 1996;Rinalducci et al, 2006;Six et al, 2007b), HL (Tamary et al, 2012) and temperature stress (Li et al, 2001;Stoitchkova et al, 2007), and thus suggests that PBS uncoupling and dismantling is a usual consequence of environmental stress in cyanobacteria. It is, however, worth noting that the highly fluorescent-free PE may also constitute a way of energy dissipation through fluorescence during a stress period.…”

Ultraviolet B-Sensitive Rice Cultivar Deficient in Cyclobutyl Pyrimidine Dimer Repair

“…GA levels, may also affect UV light responses in rice (Suge et al, 1991). Lao and Glazer (1996) detected UV-induced damage to the lightharvesting complex of the cyanobacterium Anabaena and suggested that such damage might be a major effect of UV on plants. Currently, however, the biological impact of damage to the photosynthetic apparatus is not known, nor have differences in the sensitivity of plant genotypes to such damage been demonstrated.…”

“…Li et al (1993) showed that flavonoiddeficient mutants of Arabidopsis were hypersensitive to UVB, and D' Surney et al (1993) reported that a UVsensitive soybean cultivar was deficient in the production of UV-absorbing pigments. Recently, it was suggested that UV radiation damage to the light-harvesting complex of the cyanobacterium Anabaena might be approximately 20- fold higher than the calculated dimer induction in DNA (Lao and Glazer, 1996).…”

“…Li et al (1993) showed that flavonoiddeficient mutants of Arabidopsis were hypersensitive to UVB, and D' Surney et al (1993) reported that a UVsensitive soybean cultivar was deficient in the production of UV-absorbing pigments. Recently, it was suggested that UV radiation damage to the light-harvesting complex of the cyanobacterium Anabaena might be approximately 20- fold higher than the calculated dimer induction in DNA (Lao and Glazer, 1996).Rice (Oryza sativa L.) cultivars differ in their responses to UVB radiation, with sensitive strains showing leaf browning, decreased chlorophyll production, and reduced accumulation of biomass (Teramura et al, 1991). Norin 1, which is UV-sensitive, is a grandparent of Sasanishiki, which is UV-tolerant (Kumagai and Sato, 1992).…”

Cumulative Effects of Ultraviolet Radiation and Photosyntheically Active Radiation on Phycobiliproteins of A Hot-Spring Cyanobacatrium Nostoc sp. strain HKAR-2

“…It has been reported that cyanobacterial light harvesting proteins absorb > 99 % total solar radiation (Lao and Glazer, 1996), but < 1 % UV-B radiation is enough to cause several deleterious effects on living organisms. Phycobilisomes (PBSs) constitute 50 % of the total soluble proteins in Research Article cyanobacteria.…”