Differential Gene Expression in Response to Hydrogen Peroxide and the Putative PerR Regulon ofSynechocystissp. Strain PCC 6803

We utilized a full genome cDNA microarray to identify the genes that comprise the peroxide stimulon in the cyanobacterium Synechocystis sp. strain PCC 6803. We determined that a gene (slr1738) encoding a protein similar to PerR in Bacillus subtilis was induced by peroxide. We constructed a PerR knockout strain and used it to help identify components of the PerR regulon, and we found that the regulatory properties were consistent with the hypothesis that PerR functions as a repressor. This effort was guided by finding putative PerR boxes in positions upstream of specific genes and by careful statistical analysis. PerR and sll1621 (ahpC), which codes for a peroxiredoxin, share a divergent promoter that is regulated by PerR. We found that isiA, encoding a Chl protein that is induced under low-iron conditions, was strongly induced by a short-term peroxide stress. Other genes that were strongly induced by peroxide included sigD, sigB, and genes encoding peroxiredoxins and Dsb-like proteins that have not been studied yet in this strain. A gene (slr1894) that encoded a protein similar to MrgA in B. subtilis was upregulated by peroxide, and a strain containing an mrgA knockout mutation was highly sensitive to peroxide. A number of genes were downregulated, including key genes in the chlorophyll biosynthesis pathway and numerous regulatory genes, including those encoding histidine kinases. We used PerR mutants and a thioredoxin mutant (TrxA1) to study differential expression in response to peroxide and determined that neither PerR nor TrxA1 is essential for the peroxide protective response.