Characterization of Tn1547, a composite transposon flanked by the IS16 and IS256-like elements, that confers vancomycin resistance in Enterococcus faecalis BM4281

A 64-kb genetic element harboring a vanB vancomycin-resistance (Vm(R)) gene cluster was shown to translocate from the chromosome of Enterococcus faecalis BM4281 into the hemolysin (Hly) plasmid, pIP964. Sequence analysis of the resulting junction fragments indicated that the Vm(R) genes were carried by a composite transposon, Tn1547, bounded by two distantly related insertion sequences (IS), designated IS256-like and IS16, in a direct orientation. IS256-like (1324 bp) was identical to IS256, except for two nucleotide (nt) transitions in the putative transposase gene. IS16 (1466 bp) contained a large open reading frame (ORF) that was 61% identical to the gene encoding the putative transposase of IS256. There was 58% identity between the deduced amino acid (aa) sequences of the putative transposases of IS256-like (390 aa) and IS16 (395 aa). IS16 was delineated by imperfect inverted repeats (IR) (18 out of 26 bp) which were related (23/26 bp identity) to their respective imperfect IR counterparts in IS256. The difference between the IS was not a barrier for transposition of Tn1547 which generated an 8-bp duplication at the target site. Dissemination of VanB-type resistance among enterococci results from two mechanisms: (i) large conjugative elements translocate from chromosome to chromosome following inter-strain transfer; and (ii) as described in this report, the Vm(R) genes transpose from replicon to replicon within the same strain as part of composite transposons that are internal to the conjugative elements.