Mutations in the promoter spacer region and early transcribed region increase expression of staphylococcal enterotoxin A.

The mechanism leading to increased production of staphylococcal enterotoxin type A (SEA) in mutant Staphylococcus aureus FRI722 compared within its wild-type parent strain, FRI100, was examined. Sequence analysis revealed two mutations in the upstream promoter region of FRI722 at nucleotides -28 and +3 with respect to the transcriptional initiation site at An sea translational fusion of the upstream region of FRI722 to the structural gene from FRI100 showed an increase in sea expression by Northern (RNA) analysis and in SEA production by Western (immunoblot) analysis. To independently evaluate the effect of each mutation, site-directed mutagenesis was done and revealed that each mutation was responsible for an increase in SEA production.