Staphylococcal enterotoxin B gene is associated with a discrete genetic element

Abstract

The chromosomal location of the enterotoxin B gene in Staphylococcus aureus is unknown. Southern hybridization analysis of the chromosomal DNA from several enterotoxin B (SEB)-producing strains has shown that at least 26.8 kilobases (kb) of DNA is associated with the enterotoxin B gene (entB). We have found that one end of the entB element is located approximately 1.5 kb downstream of the entB gene. The chromosomal region adjacent to this end of the entB element was found to be homologous in several SEB-producing (SEB+) and SEB-nonproducing (SEB-) S. aureus strains. The chromosomes of all the SEB+ strains studied were homologous for at least 24 kb upstream of the entB gene. Some naturally occurring SEB-strains lacked the entire entB element, while others showed variable homology to the region upstream of the entB gene. These data suggest that the entB gene is part of a discrete genetic element that is at least 26.8 kb in size.Staphylococcal enterotoxins are a group of extracellular proteins that are responsible for staphylococcal food poisoning. These enterotoxins are similar in their structure and mechanism of action and are classified into six serological groups, designated A, B, C1, C2, D, and E (2). The exact mechanism of action of these toxins is not fully understood, but they probably cause vomiting and diarrhea by an emetic action on the abdominal viscera and by inhibition of water absorption in the intestine (2, 4, 31).Staphylococcal enterotoxin B (SEB) consists of a single polypeptide chain and has a molecular weight of 28,336 (14, 17). A precursor to SEB having a molecular weight of 32,000 has been identified (17,25,32). The mature toxin consists of 239 amino acid residues and shares nucleotide and amino acid sequence homology with staphylococcal enterotoxin C1 and streptococcal pyrogenic exotoxin A (15, 34). The enterotoxin B gene (entB) has been cloned from Staphylococcus aureus S6, and its nucleotide sequence was reported (17,25).Early studies on the genetics of SEB production demonstrated that the entB gene cotransduced with the markers for methicillin resistance and tetracycline resistance at a high frequency (9). In addition, cotransduction of the entB gene with a small plasmid, pSN2, was also observed (27-29). However, the entB gene is not linked to any of these resistance markers (19,30) and has been shown to be a chromosomally linked trait (25,27,28). In addition, transformation and hybridization analyses have demonstrated that pSN2 does not play any role in SEB production (18). Earlier experiments involving transformation, transduction, and mutation analysis suggest that the entB gene is structurally unstable and possibly part of a mobile genetic element such as a phage or a transposon (4,9,18,25,28,29). In this paper we present DNA hybridization data suggesting that the entB gene is part of a discrete genetic element. MATERIALS AND METHODSBacterial strains and plasmids. The bacterial strains and plasmids used in this study are listed in Table 1.Preparation of chromosomal and plasmid DNA. Ch...

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