Nucleotide sequence of the type C3 staphylococcal enterotoxin gene suggests that intergenic recombination causes antigenic variation

The nucleotide sequence of the structural gene for staphylococcal enterotoxin type C3 (entC3) was determined. This gene contains 798-base-pair open reading frame that encodes a protein of 266 amino acid residues. Sequence analysis suggests that staphylococcal enterotoxin type C3 is synthesized in a precursor form that is processed to yield a mature extracellular form of 238 amino acid residues (molecular weight, 27,438). The entC3 gene is closely related to the gene for staphylococcal enterotoxin type C1, with 98% nucleotide sequence identity. Sequence comparisons between the entC3, entC1, and entB genes suggest that an ancestral entC1-like gene was formed by recombination between the entC3 and entB genes.