The genomes of the cyanobacteria Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 encode five and six open reading frames (ORFs), respectively, with similarity to peroxide-detoxifying peroxiredoxins (Prx). In addition to one highly conserved gene each for 2-Cys Prx and 1-Cys Prx, the Synechocystis sp. PCC 6803 genome contains one TypeII Prx and two PrxQ-like ORFs, while Synechococcus elongatus PCC 7942 has four PrxQ-like ORFs. The transcript regulation of all these bioinformatically identified genes was analysed under selected stress conditions, i.e. light limitation and light stress, hydrogen peroxide, methylviologen, salinity, as well as nitrogen- and iron-deficiency. The results on specific time- and stress-dependent regulation of transcript amounts suggest conserved as well as variable functions of these putative Prx-s in antioxidant defence. The results are discussed in the context of evolution and physiological function, particularly in relation to photosynthesis.
The genome of Synechococcus elongatus PCC 7942 encodes six peroxiredoxins (Prx). Single genes are present each for a 1-Cys Prx and a 2-Cys Prx, while four genes code for PrxQ-like proteins (prxQ-A1, -A2, -A3 and B). Their transcript accumulation varies with growth conditions in a gene-speciWc manner (Stork et al. in J Exp Bot 56:3193-3206, 2005). To address their functional properties, members of the prx gene family were produced as recombinant proteins and analysed for their peroxide detoxiWcation capacity and quaternary structure by size exclusion chromatography. Independent of the reduction state, the 2-Cys Prx separated as oligomer, the 1-Cys Prx as dimer and the PrxQ-A1 as monomer. PrxQ-A2 was inactive in our assays, 1-Cys Prx activity was unaVected by addition of TrxA, while all others were stimulated to a variable extent by addition of E. coli thioredoxin. Sensitivity towards cumene hydroperoxide treatment of E. coli BL21 cells expressing the cyanobacterial PrxQ-A1 to A3 proteins was greatly reduced, while expression of the other Prx had no eVect. The study shows diVerentiation of Prx functions in S. elongatus PCC 7942 which is discussed in relation to potential roles in site-and stress-speciWc defence.
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