Regulation of the enterotoxin B gene in Staphylococcus aureus

The enterotoxin B gene of Staphylococcus aureus encodes a single mRNA of about 900 nucleotides. To identify the DNA sequences involved in transcription of the enterotoxin B gene, the transcription initiation and termination sites were determined by nuclease S1 protection experiments. Determination of the enterotoxin B mRNA and protein levels from a number of toxin-producing strains showed that strains that contained relatively low levels of mRNA synthesized low levels of enterotoxin B, whereas strains that carried high levels of enterotoxin B mRNA produced relatively high levels of the toxin protein. Strains carrying the cloned enterotoxin B gene secreted greatly reduced amounts of other extracellular proteins, indicating that the synthesis of several exoproteins in S. aureus is coordinately regulated. An accessory element called agr has been suggested to be involved in the regulation of several exoprotein genes in S. aureus. When the cloned enterotoxin B gene was introduced into strain ISP546 in which the agr element has been inactivated, reduced levels of both enterotoxin B and enterotoxin B mRNA were found. Our results suggest that the enterotoxin B gene is regulated at the transcriptional level and that the agr element plays a role in this regulation.