The mammalian high mobility group proteins HMG1 and HMG2 are abundant, 
chromatin-associated proteins whose cellular function is not known. In this 
study we show that these proteins can substitute for the prokaryotic DNA-bending 
protein HU in promoting the assembly of the Hin invertasome, an intermediate 
structure in Hin-mediated site-specific DNA inversion. Formation of this complex 
requires the assembly of the Hin recombinase, the Fis protein, and three 
cis-acting DNA sites, necessitating the looping of intervening DNA segments. 
Invertasome assembly is strongly stimulated by HU or HMG proteins when one of 
these segments is shorter than 104 bp. By use of ligase-mediated circularization 
assays, we demonstrate that HMG1 and HMG2 can bend DNA extremely efficiently, 
forming circles as small as 66 bp, and even 59-bp circles at high HMG protein 
concentrations. In both invertasome assembly and circularization assays, 
substrates active in the presence of HMG1 contain one less helical turn of DNA 
compared with substrates active in the presence of HU protein. Analysis of 
different domains of HMG1 generated by partial proteolytic digestion indicate 
that DNA-binding domain B is sufficient for both bending and invertasome 
assembly. We suggest that an important biological function of HMG1 and HMG2 is 
to facilitate cooperative interactions between cis-acting proteins by promoting 
DNA flexibility. A general role for HMG1 and HMG2 in chromatin structure is also 
suggested by their ability to wrap DNA duplexes into highly compact forms.