Plasmid-chromosomal transition of genes important in staphylococcal enterotoxin B expression

Experiments were performed to further elucidate the genetic mechanisms underlying the synthesis of staphylococcal enterotoxin B (SEB). Our laboratory has previously shown that, in strains of Staphylococcus aureus which harbor a 1.15-megadalton plasmid (pentB or pSN2), the plasmid appears to be required for SEB synthesis; in other S. aureus strains, designated chromosomal SEB producers, this 1.15-megadalton plasmid is conspicuously absent. We report here than in both Bacillus subtilis minicells and a coupled translational assay, pSN2 codes for a polypeptide of 18,000 daltons. This product is not immunologically reactive with purified anti-SEB globulin. Nevertheless, pSN2 is necessary but not sufficient for SEB synthesis in strains which harbor the plasmid. Further, the data provide a reasonable link between plasmid-bearing and chromosomal SEB producers: transformational analysis indicates that both require functions specified (in plasmid-bearing strains) by pSN2 for SEB synthesis. The combined genetic and biochemical data suggest that pSN2 is not the reservoir for the SEB structural gene, but that the pSN2-specific functions required for SEB synthesis are regulatory in nature.