Abstract:A cDNA clone of 1081 bp encoding a second putative superoxide dismutase (SOD) from diatom Thallassiosira weissflogii was cloned by the polymerase chain reaction technique. The cDNA encodes a protein of 286 amino acid residues. Alignment of the truncated SOD sequence containing 217 amino acid residues with Mn−SODs from Vibrio mimicus and Escherichia coli, as well as two Fe−SODs from E. coli and Photobacterium leiognathi, this SOD showed greater homology to Mn−SOD. The residues required to coordinate the manganese ion were conserved in all reported Mn−SOD. The recombinant SOD has a half life of deactivation of 14.7 min at 65 °C. Its thermal inactivation rate constant Kd was 3.21 × 10-2 min-1. The enzyme was stable in a broad pH range from 4 to 12. The presence of imidazole (up to 0.8 M) and sodium dodecylsulfate (up to 4%) had little effect on the enzyme's activity. The atomic absorption spectrometric assay showed the presence of 0.3 atom of iron/manganese (2:1) in each SOD subunit. Reconstituted activity suggested that diatom SOD was cambialistic Fe/Mn−SOD.