Abstract
BackgroundCyanobacteria, progenitors of plant chloroplasts, provide a suitable model system for plants to study adaptation towards different abiotic stresses. Genome of the filamentous, heterocystous, nitrogen-fixing cyanobacterium Anabaena PCC7120 harbours a single gene (alr4641) encoding a typical 2-Cys-Peroxiredoxins (2-Cys-Prxs). 2-Cys-Prxs are thiol-based peroxidases that also function as molecular chaperones in plants and other systems. The Alr4641 protein from Anabaena PCC7120 shows high level biochemical similarities with the plant 2-Cys-Prx. The physiological role played by the Alr4641 protein in Anabaena was addressed in this study.ResultsIn Anabaena PCC7120, alr4641 transcript /Alr4641 protein was induced in response to abiotic stresses and its promoter was active in the vegetative cells as well as heterocysts. The wild-type Alr4641 protein or Alr4641 lacking the peroxidatic cysteine (Alr4641C56S) or the resolving cysteine (Alr4641C178S) existed as higher oligomers in their native form. The wild-type or the mutant Alr4641 proteins showed similar chaperone activity, but only the wild-type protein exhibited peroxidase activity indicating that unlike peroxidase activity, chaperone activity was not dependent on cysteines. In contrast to other 2-Cys-Prxs, chaperone/peroxidase activity of Alr4641 was dependent on its redox state and not oligomerization status. Alr4641 could protect plasmid DNA from oxidative damage and physically associate with NADPH-dependent thioredoxin reductase (NTRC). Like 2-Cys-Prxs from plants (e.g. rice), Alr4641 could detoxify various peroxides using NTRC as reductant. On exposure to H2O2, recombinant Anabaena PCC7120 strain over-expressing Alr4641 (An4641+) showed reduced content of reactive oxygen species (ROS), intact photosynthetic functions and consequently better survival than the wild-type Anabaena PCC7120, indicating that Alr4641 can protect Anabaena from oxidative stress.ConclusionsThe peroxidase/chaperone function of Alr4641, its inherent transcriptional/translational induction under different abiotic stresses and localization in both vegetative cells and heterocysts could be an adaptive strategy to battle various oxidative stresses that Anabaena encounters during its growth. Moreover, the recombinant Anabaena strain over expressing Alr4641 showed higher resistance to oxidative stress, suggesting its potential to serve as stress-tolerant biofertilizers in rice fields.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0444-2) contains supplementary material, which is available to authorized users.
Highlights
Cyanobacteria, progenitors of plant chloroplasts, provide a suitable model system for plants to study adaptation towards different abiotic stresses
The wild-type Anabaena PCC7120 cells were treated with methyl viologen (MV), hydrogen peroxide (H2O2) or tertiary butyl hydroperoxide (t-Bx) for 1 h
Western blotting followed by immunodetection with the Alr4641 antiserum revealed salt, sucrose and MV to increase the content of the Alr4641 protein as compared to the untreated control cells (Figure 1D)
Summary
Cyanobacteria, progenitors of plant chloroplasts, provide a suitable model system for plants to study adaptation towards different abiotic stresses. 2-Cys-Prxs are thiol-based peroxidases that function as molecular chaperones in plants and other systems. The Alr4641 protein from Anabaena PCC7120 shows high level biochemical similarities with the plant 2-Cys-Prx. The physiological role played by the Alr4641 protein in Anabaena was addressed in this study. Prxs have highly conserved cysteine residues, peroxidatic cysteine (Cp) and resolving cysteine (Cr), which are essential for peroxidase activity. Based on their catalytic mechanisms and the presence of conserved cysteine residues, Prxs are classified into three groups, namely, typical 2-Cys-Prx, atypical 2-Cys Prx (which are subdivided into type II Prx and PrxQ) and 1-Cys-Prx [3]. The typical 2-Cys-Prxs are functionally conserved across diverse organisms and form the largest group of peroxiredoxins. In typical 2-Cys-Prxs, Cp is present near N-terminus
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