This report describes the isolation, sequencing, and functional analysis of the sodA gene, encoding Mn-superoxide dismutase, from Streptococcus thermophilus AO54. The gene was found to encode a 201 amino acid polypeptide with 88 and 83% identity to SodA from Streptococcus mutans and Streptococcus agalacticae, respectively. Primer extension analysis revealed a transcriptional start site 27 nucleotides upstream of initiation codon. The gene was expressed in Escherichia coli and was able to rescue the growth of a sodAsodB mutant in a minimal-medium containing 10(-6)M paraquat. A sodA mutant of S. thermophilus was constructed and found to be more sensitive to aerobic growth than its parent strain. Supplementing the medium with MnCl(2) improved the growth of the mutant, only under microaerophilic conditions. The results suggest that sodA is essential for the aerobic growth of S. thermophilus. In the absence of functional SodA, manganese ions may provide partial protection against oxygen toxicity.