The prokaryotic protein HU functions as an accessory factor in many different 
biochemical reactions. We have characterized the role of HU in assembling the 
invertasome, an intermediate nucleoprotein complex involved in Hin-mediated 
site-specific recombination. Formation of this complex requires the looping of 
intervening DNA segments between sites bound by the Hin recombinase and the Fis 
protein. HU stimulates this process on substrates containing intervening 
segments of length < 100 bp. Characterization of the activity of HU in 
Hin-mediated recombination in vitro and in vivo yields evidence that its role in 
this reaction is primarily to facilitate the looping of the intervening DNA 
segment. By using this reaction as an assay, we identify proteins from mammals, 
yeast, trypanosomes, and wheat which can fulfill the same function in vitro. 
Using ligase-mediated circularization of short DNA fragments we also show that 
HU, the high mobility group (HMG) 1 and 2 proteins from mammals, and a protein 
from yeast can bend DNA extremely efficiently. These results support the view 
that this ubiquitous class of proteins enhance the assembly of nucleoprotein 
complexes under conditions of limited DNA flexibility.