The uvrA, uvrB, and uvrC genes of Escherichia coli control the initial steps of 
nucleotide excision repair. The uvrC gene product is involved in at least one of 
the dual incisions produced by the UvrABC complex. Using single-stranded (ss) 
DNA affinity chromatography, we have separated two forms of UvrC from both 
wild-type E. coli cells and overproducing cells. UvrCI elutes at 0.4 M KCl, and 
UvrCII elutes at 0.6 M KCl. In general, both forms, in the presence of UvrA and 
UvrB, actively incise UV-irradiated and CC-1065-modified DNA in the same 
fashion; i.e., they incise six to eight nucleotides 5' to and three to five 
nucleotides 3' to a photoproduct or a CC-1065-N3-adenine adduct. They produce 
different incisions, however, at a CC-1065-N3-adenine adduct in the sequence 
5'-GATTACG- present in the MspI-BstNI 117 bp fragment of M13mp1. UvrABCI incises 
at both the 5' and 3' sides of the adduct (UvrABCI cut), while UvrABCII incises 
only at the 5' side (UvrABCII cut). Mixing UvrCI and UvrCII results in both 
UvrABCI and UvrABCII cuts, and the levels of these two types of cutting are 
proportional to the amount of UvrCI and UvrCII. DNase I footprints of the 
MspI-BstNI 117 bp DNA fragment containing a site-directed CC-1065-adenine adduct 
at the 5'-GATTACG- site show that UvrCII, but not UvrCI, binds to the adduct 
site. Furthermore, the pattern of DNase I footprints induced by UvrCII binding 
differs from the pattern of the footprints induced by UvrA, UvrAB, and UvrABCI 
binding. Interestingly, while the presence of unirradiated DNA enhances the 
efficiency of UvrABCII in incising UV-irradiated DNA, it does not enhance 
UvrABCII incision of the CC-1065-N3-adenine adduct formed at 5'-GATTACG-. These 
results show that two different forms of UvrC differ in DNA binding properties 
as well as incision modes at some kinds of DNA damage.