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Journal of Bacteriology, September 2002, p. 5096-5103, Vol. 184, No. 18
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.18.5096-5103.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Differential Expression and Localization of Mn and Fe Superoxide Dismutases in the Heterocystous Cyanobacterium Anabaena sp. Strain PCC 7120

Tao Li,1 Xu Huang,1 Ruanbao Zhou,1 Yingfang Liu,1 Bin Li,1 Chris Nomura,2 and Jindong Zhao1,3*

State Key Lab of Plant Genetic and Protein Engineering,3 College of Life Sciences, Peking University, Beijing 100871, China,1 Department of Biochemistry, Pennsylvania State University, University Park, Pennsylvania 168022

Received 12 March 2002/ Accepted 14 June 2002

Superoxide dismutases (Sods) play very important roles in preventing oxidative damages in aerobic organisms. The nitrogen-fixing heterocystous cyanobacterium Anabaena sp. strain PCC 7120 has two Sod-encoding genes: a sodB, encoding a soluble iron-containing Sod (FeSod), and a sodA, encoding a manganese-containing Sod (MnSod). The FeSod was purified and characterized. A recombinant FeSod was also obtained by overproduction in Escherichia coli. Immunoblot study of the FeSod during induction of heterocyst differentiation showed that the cells produced six- to eightfold more FeSod 8 h after a shift from a nitrogen-replete condition to a nitrogen-depleted condition. However, the amount of FeSod protein in filaments with mature heterocysts was the same as that in filaments grown with combined nitrogen. Superoxide anion-generating chemicals such as methyl viologen did not induce upregulation of the sodB gene expression. The predicted preprotein of the sodA gene has a leader peptide and a motif for membrane attachment at the N terminus of the mature protein. Activity staining after gel electrophoresis of the purified thylakoid membranes showed that most of the MnSod in Anabaena sp. strain PCC 7120 was located on thylakoids toward the lumenal side. Expression of the sodA gene in E. coli shows that the leader peptide was required for its activity and the membrane localization of the MnSod. Northern hybridization detected one 0.82-kb transcript of sodA. The sodA gene was upregulated by methyl viologen, whereas its amount was unchanged during heterocyst differentiation. Immunoblotting and activity staining showed that isolated heterocysts contained a lower but still significant amount of FeSod, suggesting that its function is required in heterocysts. No MnSod was observed in isolated heterocysts. These results show that the two different Sod proteins have differentiated roles in Anabaena sp. strain PCC 7120.

* Corresponding author. Mailing address: College of Life Sciences, Peking University, Beijing 100871, People's Republic of China. Phone: 86-10-6275-6421. Fax: 86-10-6275-1526. E-mail: jzhao{at}pku.edu.cn.

Journal of Bacteriology, September 2002, p. 5096-5103, Vol. 184, No. 18
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.18.5096-5103.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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