Genetic factors associated with enhanced bla KPC expression in Tn3/Tn4401 chimeras.

The expression of the bla KPC gene plays a key role in carbapenem resistance in Enterobacteriaceae However, the genetic regulators of the bla KPC gene have not been completely elucidated, especially the genes in Tn 3 -Tn 4401 chimeras. Two novel Tn 3 -Tn 4401 chimera isoforms were characterized in our hospital: isoform A (CTA), which harbors a 121-bp deletion containing the PX promoter and was present in 22.6% (54/239) of isolates, and isoform C (CTC), which harbors a 624-bp insertion and a P1 promoter deletion and was present in only 1 isolate. The carbapenem minimum inhibitory concentrations (MICs) of both isoforms were 2-fold or more higher than those of the wild-type (Tn 3 -Tn 4401 chimera, CTB), and bla KPC was most highly expressed in CTA. Bioinformatics and 5' rapid amplification of cDNA ends (5' RACE) experiments indicated a novel strong putative promoter, PY, at the 3' end of the IS Kpn8 gene. PY mutation nearly abrogated bla KPC expression ( p <0.01) and restored carbapenem susceptibility in all 3 isoforms. Although the mutation of PX or P1 halved bla KPC expression in CTB ( p <0.05), PX deletion caused a 68% increase in bla KPC expression ( p =0.037) in CTA. The level of bla KPC mRNA in CTC was 8-fold higher than that in InCTC, which harbors P1 ( p =0.011). These results suggest that PY is a core promoter of the bla KPC gene in the chimeras and that the deletion of the PX and P1 promoters enhanced gene expression in CTA and CTC, respectively.