Characterization of four superoxide dismutase genes from a filamentous cyanobacterium

By using an oligonucleotide probe constructed from a conserved region of amino acids located in the carboxyl-terminal end of superoxide dismutase (SOD) proteins, four SOD genes were cloned from the cyanobacterium Plectonema boryanum UTEX 485. One of these genes, designated sodB, encoded an FeSOD enzyme, while the remaining three genes, designated sodA1, sodA2, and sodA3, encoded MnSOD enzymes. To investigate the expression of these four genes, total cellular RNA was isolated from P. boryanum UTEX 485 cells grown under various conditions and RNA gel blot analysis was carried out. Results indicated that sodB and sodA1 were constitutively expressed, although sodB expression was partially repressed in cells grown under conditions of iron stress. sodA2 transcripts, which were not detectable in control cells, accumulated to high levels in cells treated with methyl viologen or in cells grown under conditions of iron or nitrogen stress. However, under microaerobic conditions, iron and nitrogen stress failed to induce sodA2, indicating that multiple factors affect the regulation of sodA2. While discrete transcripts were not detected for sodA3, hybridization was observed under a number of conditions, including those which increased the accumulation of sodA2 transcripts. Additionally, there were high levels of the sodA3 transcript detected in a P. boryanum UTEX 485 mutant strain resistant to methyl viologen treatment.