UvrB plays a key role in bacterial nucleotide excision repair. It is the 
ultimate damage-binding protein that interacts with both UvrA and UvrC. The 
oligomeric state of UvrB and the UvrAB complex have been subject of debate for a 
long time. Using fluorescence resonance energy transfer (FRET) between GFP and 
YFP fused to the C-terminal end of Escherichia coli UvrB, we unambiguously show 
that in solution two UvrB subunits bind to UvrA, most likely as part of a 
UvrA2B2 complex. This complex is most stable when both UvrA and UvrB are in the 
ATP-bound form. Analysis of a truncated form of UvrB shows that binding to UvrA 
promotes dimerization of the two C-terminal domain 4 regions of UvrB. The 
presence of undamaged DNA leads to dissociation of the UvrA2B2 complex, but when 
the ATPase site of UvrB is inactivated, the complex is trapped on the DNA. When 
the complex is bound to a damaged site, FRET between the two UvrB subunits could 
still be detected, but only as long as UvrA remains associated. Dissociation of 
UvrA from the damage-bound UvrB dimer leads to the reduction of the magnitude of 
the FRET signal, indicating that the domain 4 regions no longer interact. We 
propose that the UvrA-induced dimerization of the domain 4 regions serves to 
shield these domains from premature UvrC binding. Only after specific binding of 
the UvrB dimer to a damaged site and subsequent release of UvrA is the contact 
between the domain 4 regions broken, allowing recruitment of UvrC and subsequent 
incisions.