It is generally accepted that the damage recognition complex of nucleotide 
excision repair in Escherichia coli consists of two UvrA and one UvrB molecule, 
and that in the preincision complex UvrB binds to the damage as a monomer. Using 
scanning force microscopy, we show here that the damage recognition complex 
consists of two UvrA and two UvrB subunits, with the DNA wrapped around one of 
the UvrB monomers. Upon binding the damage and release of the UvrA subunits, 
UvrB remains a dimer in the preincision complex. After association with the UvrC 
protein, one of the UvrB monomers is released. We propose a model in which the 
presence of two UvrB subunits ensures damage recognition in both DNA strands. 
Upon binding of the UvrA(2)B(2) complex to a putative damaged site, the DNA 
wraps around one of the UvrB monomers, which will subsequently probe one of the 
DNA strands for the presence of a lesion. When no damage is found, the DNA will 
wrap around the second UvrB subunit, which will check the other strand for 
aberrations.