  In spite of the nanoscale and single-molecule insights into how nucleoid
associated proteins (NAPs) interact with DNA, their role in modulating the
mesoscale viscoelasticity of the entangled genome in vivo has been overlooked
so far. By combining microrheology and molecular dynamics simulation we find
that the important NAP called Integration Host Factor (IHF) lowers the
viscosity of entangled $\lambda$DNA 20-fold at physiological concentrations and
stoichiometries. We argue that IHF may act as a "genomic fluidiser", reducing
the effective viscosity of the nucleoid $\sim$200-fold. Our results suggest a
previously unappreciated key role of IHF in regulating DNA dynamics and
re-organisation in vivo
