The Role of Peroxiredoxins in Oxygenic Photosynthesis of Cyanobacteria and Higher Plants: Peroxide Detoxification or Redox Sensing?

Microcystin aids in cold temperature acclimation: Differences between a toxic Microcystis wildtype and non-toxic mutant

For Microcystis aeruginosa PCC 7806, temperature decreases from 26 °C to 19 °C double the microcystin quota per cell during growth in continuous culture. Here we tested whether this increase in microcystin provided M. aeruginosa PCC 7806 with a fitness advantage during colder-temperature growth by comparing cell concentration, cellular physiology, reactive oxygen species damage, and the transcriptomics-inferred metabolism to a non-toxigenic mutant strain M. aeruginosa PCC 7806 ΔmcyB. Photo-physiological data combined with transcriptomic data revealed metabolic changes in the mutant strain during growth at 19 °C, which included increased electron sinks and non-photochemical quenching. Increased gene expression was observed for a glutathione-dependent peroxiredoxin during cold treatment, suggesting compensatory mechanisms to defend against reactive oxygen species are employed in the absence of microcystin in the mutant. Our observations highlight the potential selective advantages of a longer-term defensive strategy in management of oxidative stress (i.e., making microcystin) vs the shorter-term proactive strategy of producing cellular components to actively dissipate or degrade oxidative stress agents.

Genome-wide analysis of putative peroxiredoxin in unicellular and filamentous cyanobacteria

BACKGROUND

Cyanobacteria are photoautotrophic prokaryotes with wide variations in genome sizes and ecological habitats. Peroxiredoxin (PRX) is an important protein that plays essential roles in protecting own cells against reactive oxygen species (ROS). PRXs have been identified from mammals, fungi and higher plants. However, knowledge on cyanobacterial PRXs still remains obscure. With the availability of 37 sequenced cyanobacterial genomes, we performed a comprehensive comparative analysis of PRXs and explored their diversity, distribution, domain structure and evolution.

RESULTS

Overall 244 putative prx genes were identified, which were abundant in filamentous diazotrophic cyanobacteria, Acaryochloris marina MBIC 11017, and unicellular cyanobacteria inhabiting freshwater and hot-springs, while poor in all Prochlorococcus and marine Synechococcus strains. Among these putative genes, 25 open reading frames (ORFs) encoding hypothetical proteins were identified as prx gene family members and the others were already annotated as prx genes. All 244 putative PRXs were classified into five major subfamilies (1-Cys, 2-Cys, BCP, PRX5_like, and PRX-like) according to their domain structures. The catalytic motifs of the cyanobacterial PRXs were similar to those of eukaryotic PRXs and highly conserved in all but the PRX-like subfamily. Classical motif (CXXC) of thioredoxin was detected in protein sequences from the PRX-like subfamily. Phylogenetic tree constructed of catalytic domains coincided well with the domain structures of PRXs and the phylogenies based on 16s rRNA.

CONCLUSIONS

The distribution of genes encoding PRXs in different unicellular and filamentous cyanobacteria especially those sub-families like PRX-like or 1-Cys PRX correlate with the genome size, eco-physiology, and physiological properties of the organisms. Cyanobacterial and eukaryotic PRXs share similar conserved motifs, indicating that cyanobacteria adopt similar catalytic mechanisms as eukaryotes. All cyanobacterial PRX proteins share highly similar structures, implying that these genes may originate from a common ancestor. In this study, a general framework of the sequence-structure-function connections of the PRXs was revealed, which may facilitate functional investigations of PRXs in various organisms.

The dual function of plant peroxiredoxins in antioxidant defence and redox signaling

With 8 to 10 members, the peroxiredoxin gene family of each higher plant with known genome sequence is larger than in other eukaryotes. Likewise, the complexity of reductive regenerants is very high, e.g. the chloroplast 2-Cys Prx is reduced by various thioredoxins, cyclophilin Cyp20-3, the drought induced CDSP32 and the NADPH-dependent reductant NTRC. In the light of the apparent versatility of the peroxiredoxin system in plants, its specific and important functions in antioxidant defence, photosynthesis and stress adaptation, the review attempts a survey of present day knowledge on plant peroxiredoxins, their biochemical features and transcript regulation, as well as their function in photosynthesis, development, stress response and pathogenesis. The emerging evidence for plant Prx function in cell signaling is summarized.

Peroxiredoxin Q of Arabidopsis thaliana is attached to the thylakoids and functions in context of photosynthesis

Peroxiredoxin Q (Prx Q) is one out of 10 peroxiredoxins encoded in the genome of Arabidopsis thaliana, and one out of four that are targeted to plastids. Peroxiredoxin Q functions as a monomeric protein and represents about 0.3% of chloroplast proteins. It attaches to the thylakoid membrane and is detected in preparations enriched in photosystem II complexes. Peroxiredoxin Q decomposes peroxides using thioredoxin as an electron donor with a substrate preference of H(2)O(2) > cumene hydroperoxide >> butyl hydroperoxide >> linoleoyl hydroperoxide and insignificant affinity towards complex phospholipid hydroperoxide. Plants with decreased levels of Prx Q did not have an apparently different phenotype from wildtype at the plant level. However, similar to antisense 2-cysteine (2-Cys) Prx plants [Baier, M. et al. (2000)Plant Physiol., 124, 823-832], Prx Q-deficient plants had a decreased sensitivity to oxidants in a leaf slice test as indicated by chlorophyll a fluorescence measurements. Increased fluorescence ratios of photosystem II to I at 77 K and modified transcript levels of plastid- and nuclear-encoded proteins show that regulatory mechanisms are at work to compensate for the lack of Prx Q. Apparently Prx Q attaches to photosystem II and has a specific function distinct from 2-Cys peroxiredoxin in protecting photosynthesis. Its absence causes metabolic changes that are sensed and trigger appropriate compensatory responses.