Molecular characterization and expression analysis of the superoxide dismutase gene from Streptococcus agalactiae

We have cloned and sequenced a 3103-bp DNA fragment carrying the gene encoding the Mn-SOD from Streptococcus agalactiae NEM318 serotype III. This DNA fragment contained four orfs that have the same polarity of transcription. Orf1 was truncated by molecular cloning and the corresponding 228-aa-long polypeptide did not exhibit any significant homology with other cognate proteins. Orf2 encodes a protein of 345 aa that displays some similarity (29% identity) with the YqeN peptide of Bacillus subtilis, the function of which is unknown. Orf3 encodes the 202-aa-long Mn-SOD which was functionally expressed in Escherichia coli. Orf4 was also truncated by molecular cloning and encodes 99 aa of the N-terminal moiety of a protein that displays significant homology (40% f identity) with the antiterminator LicT of B. subtilis. Transcriptional analysis revealed that the sodA gene of S. agalactiae NEM318 was transcribed monocistronically from a promoter, the activity of which is neither regulated by pH, O2, nor CO2 concentrations of the culture medium. Analysis by high resolution agarose gel electrophoresis of the AluI DNA polymorphism of the sodA locus in wild-type strains of S. agalactiae belonging to serogroups I, II, or III revealed no detectable difference.