H-NS is one of the two most abundant proteins in the bacterial nucleoi
d and influences the expression of a number of genes. We have studied
the interaction of H-NS with DNA; purified H-NS was demonstrated to co
nstrain negative DNA supercoils in vitro. This provides support for th
e hypothesis that H-NS influences transcription via changes in DNA top
ology, and is evidence of a structural role for H-NS in bacterial chro
matin. The effects of H-NS on topology were only observed at sub-satur
ating concentrations of the protein. In addition, a preferred binding
site on DNA was identified by DNase I footprinting at sub-saturating H
-NS concentrations. This site corresponded to a curved sequence elemen
t which we previously showed, by in vivo studies, to be a site at whic
h H-NS influences transcription of the proU operon. When present in sa
turating concentrations, H-NS did not constrain supercoils and bound t
o DNA in a sequence-independent fashion, covering all DNA molecules fr
om end to end, suggesting that H-NS may form distinct complexes with D
NA at different H-NS:DNA ratios. The data presented here provide direc
t support for the hypothesis that H-NS acts at specific sites to influ
ence DNA topology and, hence, transcription.