Roles of the cyanobacterial isiABC operon in protection from oxidative and heat stresses

Abstract

The expression of the isiAB operon, which encodes CP43′ and flavodoxin, is induced under oxidative‐ or heat‐stress conditions as well as iron‐limited conditions. The aim of this study is to elucidate their roles for acquisition of tolerance by cells against oxidative or heat stress with using the isiA interruption and the isiB deletion mutants of Synechocystis sp. PCC 6803. In the former mutant, the isiA messenger RNA was truncated and the isiB transcription was totally abolished. Northern blot analysis indicated that an open reading frame sll0249 located immediate downstream of isiB is cotranscribed with isiAB under iron‐limited conditions. Thus, we designated the gene as isiC and constructed an isiC deletion mutant to test polar effects of the isiA and isiB disruptions on the isiC expression. All the mutants were highly sensitive to oxidative stress caused by the presence of methyl viologen, indicating an essential role of the operon, especially isiC, under oxidative stress. The isiA mutant was much more susceptible to the sublethal heat stress than the other mutants. Determination of the survival rate after a lethal temperature treatment confirmed the essential role of the isiA gene for the thermal stress management. We found that light conditions immediately after the lethal temperature treatment have a great effect on the survival rate of the wild‐type and the isiA mutant strains. We suggest that IsiA and IsiB may play a role in optimizing the light energy supply to avoid cellular damage and utilizing the light energy in the post‐heat‐stress recovery process.