Photo-oxidative stress in Rhodobacter sphaeroides: protective role of carotenoids and expression of selected genes

Abstract

In Rhodobacter sphaeroides, carotenoids are essential constituents of the photosynthetic apparatus and are assumed to prevent the formation of singlet oxygen by quenching of triplet bacteriochlorophyll a (BChl a) in vivo. It was shown that small amounts of singlet oxygen are generated in vivo by incubation of R. sphaeroides under high light conditions. However, growth and survival rates were not affected. Higher amounts of singlet oxygen were generated by BChl a in a carotenoid-deficient strain and led to a decrease in growth and survival rates. The data support earlier results on the pivotal role of carotenoids in the defence against stress caused by singlet oxygen. Results obtained under photo-oxidative stress conditions with strains impaired in carotenoid synthesis suggest that sphaeroidene and neurosporene provide less protection against methylene-blue-generated singlet oxygen than sphaeroidenone in vivo. Despite their protective function against singlet oxygen, relative amounts of carotenoids did not accumulate in R. sphaeroides wild-type cultures under photo-oxidative stress, and relative mRNA levels of phytoene dehydrogenase and sphaeroidene monooxygenase did not increase. In contrast, singlet oxygen specifically induced the expression of glutathione peroxidase and a putative Zn-dependent hydrolase, but mRNA levels of hydrogen-peroxide-degrading catalase E were not significantly affected by photo-oxidative stress. Based on these results, it is suggested that singlet oxygen acts as a specific signal for gene expression in R. sphaeroides. Presumably transcriptional regulators exist to specifically induce the expression of genes involved in the response to stress caused by singlet oxygen.

Superoxide Generation by Chlorophyllide a Reductase of Rhodobacter sphaeroides

“…Unless quenched by carotenoid and resolved by a number of E -regulated gene products, 1 O 2 causes various damage to proteins, lipids, and DNA (72)(73)(74). Superoxide generation from COR reaction, which is readily dismutated by cytosolic Fe-SOD, might be considered electron leakage through heme.…”

The phrA gene of Rhodobacter sphaeroides encodes a photolyase and is regulated by singlet oxygen and peroxide in a σ E-dependent manner

“…It was shown previously that, in the presence of oxygen and high light, R. sphaeroides forms small amounts of singlet oxygen in vivo, which do not adversely affect cell growth (Glaeser & Klug, 2005). Higher amounts of singlet oxygen were generated by bacteriochlorophyll a in the carotenoiddeficient strain TC67, causing a decrease in growth and survival rates (Glaeser & Klug, 2005). When strain TC67(pPHUphrAlacZ) was illuminated with white light of low intensity (30 W m 22 ), the b-galactosidase activity increased about fivefold within 2 h (Fig.…”

“…Carotenoids are essential constituents of the photosynthetic apparatus and are assumed to prevent the formation of singlet oxygen by quenching of triplet bacteriochlorophyll a in vivo (Borland et al, 1989). It was shown previously that, in the presence of oxygen and high light, R. sphaeroides forms small amounts of singlet oxygen in vivo, which do not adversely affect cell growth (Glaeser & Klug, 2005). Higher amounts of singlet oxygen were generated by bacteriochlorophyll a in the carotenoiddeficient strain TC67, causing a decrease in growth and survival rates (Glaeser & Klug, 2005).…”

“…When R. sphaeroides grows in the absence of oxygen by anaerobic respiration, light has a stimulating effect on the expression of photosynthesis genes (Braatsch et al, 2002). Recent investigations on the protection of R. sphaeroides against photooxidative stress showed the involvement of s E (RSP1092) and the anti-s E factor ChrR (RSP1093) in this response (Anthony et al, 2005;Glaeser & Klug, 2005). s E is a member of the extracytoplasmatic function (ECF) family of s factors.…”

Photoprotection in intact cells of photosynthetic bacteria: quenching of bacteriochlorophyll fluorescence by carotenoid triplets

“…gelatinosus (Ranck et al 2001) and in Rba. sphaeroides (Cogdell 1978) In addition to serving as accessory light-harvesting pigments (Cogdell and Frank 1987), carotenoids may play a structural role (Lang and Hunter 1994;Fraser et al 2001;Formaggio et al 2001), and provide protection against photooxidative damage (Griffiths et al 1955;Cogdell et al 2000;Cogdell et al 2004;Glaeser and Klug 2005). The light energy absorbed by the carotenoids in the blue-green region of the visible spectrum (where BChls are weak absorbers) can be rapidly transferred to the BChls thus increasing the cross-sectional area of the spectrum available for capture by the bacteria.…”