Cloning and characterization of an Anacystis nidulans R2 superoxide dismutase gene

The E. coli iron superoxide dismutase gene (sodB) was utilized as a heterologous probe to isolate a superoxide dismutase (sod) gene from Anacystis nidulans R2. Nucleotide sequence analysis revealed a 603 bp open reading frame with deduced amino acid sequence similar to other sod genes and to cyanobacterial superoxide dismutase amino-terminal sequences. Assuming proteolytic cleavage of the initial methionine residue, the molecular mass of the mature A. nidulans R2 sodB polypeptide is 22,000 daltons. Only a single copy of the superoxide dismutase sequence was detected in the A. nidulans R2 genome using Southern hybridization. Northern hybridization analysis indicated a single, monocistronic RNA transcript of approximately 720 bases. Primer extension mapping localized the transcription start site to 46 bases upstream from the initial methionine residue. A single orientation of a 2.1 kb PstI fragment containing the entire sod gene cloned into pUC18 was able to complement E. coli sodAsodB mutants. Complementation of the E. coli mutants was based on the ability of the cells to grow aerobically on minimal glucose medium. Growth curves of the complemented E. coli sodAsodB mutants showed that these cells exhibited levels of resistance to paraquat comparable to that of the wild-type E. coli phenotype.