HU is one of the most abundant DNA binding proteins in Escherichia coli. Like 
the histones, HU is able to condense DNA in vitro and to introduce negative 
super-coiling in covalently closed circular, relaxed DNA molecules in the 
presence of topoisomerase I. HU is well conserved in all prokaryotes but 
surprisingly, it is also homologous to another E. coli DNA-binding protein, IHF. 
Contrary to HU, IHF shows sequence specificity and is much less abundant that 
HU. Both are heterodimers and all four polypeptide chains probably arose from a 
common ancestor. The question we raised was whether IHF could supply the main 
functions of HU in its absence. The answer seems to be negative for the 
following reasons. We did not observe any significant regulation of expression 
of the genes coding for HU by IHF, or vice-versa. The structures that these two 
proteins form with double-stranded or single-stranded DNA are completely 
different. Finally, overexpression of IHF does not relieve the growth defects 
observed in HU-less mutants. However, it can be speculated from our results that 
even if the two proteins are not equivalent and cannot replace one another, both 
could stimulate (or inhibit) some specific protein-DNA interactions or affect 
the DNA-binding properties of one another.