Streptococcus pneumoniae Transposon Tn1545/Tn6003 Changes to Tn6002 Due to Spontaneous Excision in Circular Form of the erm(B)-and aphA3-Containing Macrolide-Aminoglycoside-Streptothricin (MAS) Element

The macrolide-aminoglycoside-streptothricin (MAS) element, an4.2-kb insertion containing erm(B) and aphA3 resistance determinants, distinguishes Streptococcus pneumoniae transposon Tn1545/Tn6003 from Tn6002. Here, it is shown to be an unstable genetic element that, although it lacks recombinase genes, can exploit long, erm(B)-containing direct repeats acting as att sites for spontaneous excision that may result in loss. Consequent to excision, which is RecA independent, Tn1545/Tn6003 changes to Tn6002. In pneumococcal populations harboring Tn1545/Tn6003, the latter appears to coexist with Tn6002.