- Trimethoprim resistance transposon Tn4003 from Staphylococcus aureus encodes genes for a dihydrofolate reductase and thymidylate synthetase flanked by three copies of IS257. Rouch, D.A., Messerotti, L.J., Loo, L.S., Jackson, C.A., Skurray, R.A. Mol. Microbiol. (1989)