An IS257-Derived Hybrid Promoter Directs Transcription of a tetA(K) Tetracycline Resistance Gene in theStaphylococcus aureus Chromosomal mecRegion
ABSTRACT Transcription of the tetA(K) tetracycline resistance determinant encoded by an IS257-flanked cointegrated copy of a pT181-like plasmid, located within the chromosomal mecregion of a methicillin-resistant Staphylococcus aureusisolate, has been investigated. The results demonstrated that transcription of tetA(K) in this strain is directed by both an IS257-derived hybrid promoter, which is stronger than the native tetA(K) promoter in the autonomous form of pT181, and a complete outwardly directed promoter identified within one end of IS257. Despite lower gene dosage, the chromosomal configuration was shown to afford a higher level of resistance than that mediated by pT181 in an autonomous multicopy state. Furthermore, competition studies revealed that a strain carrying the chromosomaltetA(K) determinant exhibited a higher level of fitness in the presence of tetracycline but not in its absence. This finding suggests that tetracycline has been a selective factor in the emergence of strains carrying a cointegrated pT181-like plasmid in their chromosomes. The results highlight the potential of IS257to influence the expression of neighboring genes, a property likely to enhance its capacity to mediate advantageous genetic rearrangements.