The effect of host-encoded nucleoid proteins on transposition: H-NS influences targeting of both IS903 and Tn10

Nucleoid proteins are small, abundant, DNA-binding proteins that profoundly affect the local and global structure of the chromosome, and play a major role in gene regulation. Although several of these proteins have been shown to enhance assembly of transpososomes before initiating transposition, no systematic survey has been carried out examining the in vivo role(s) of these proteins in transposition. We have examined the requirement of the six most abundant nucleoid proteins in transposition for three different transposons, IS903, Tn10 and Tn552. Most notably, H-NS was required for efficient transposition of all three elements in a papillation assay, suggesting a general role for H-NS in bacterial transposition. Further studies indicated that H-NS was exerting its effect on target capture. Targeting preferences for IS903 into the Escherichia coli chromosome were dramatically altered in the absence of H-NS. In addition, the alterations observed in the IS903 target profile emphasized the important role that H-NS plays in chromosome organization. A defect in target capture was also inferred for Tn10, as an excised transposon fragment, a precursor to target capture, accumulated in in vivo induction assays. Furthermore, a transposase mutant that is known to increase target DNA bending and to relax target specificity eliminated this block to target capture. Together, these results imply a role for H-NS in target capture, either by providing regions of DNA more accessible to transposition or by stabilizing transpososome binding to captured targets immediately before strand transfer.