Transcription is coupled to repair in Escherichia coli and in humans. Proteins 
encoded by the mfd gene in E. coli and by the ERCC6/CSB gene in humans, both of 
which possess the so-called helicase motifs, are required for the coupling 
reaction. It has been shown that the Mfd protein is an ATPase but not a helicase 
and accomplishes coupling, in part, by disrupting the ternary complex of E. coli 
RNA polymerase stalled at the site of DNA damage. In this study we overproduced 
the human CSB protein using the baculovirus vector and purified and 
characterized the recombinant protein. CSB has an ATPase activity that is 
stimulated strongly by DNA; however, it neither acts as a helicase nor does it 
dissociate stalled RNA polymerase II, suggesting a coupling mechanism in humans 
different from that in prokaryotes. CSB is a DNA-binding protein, and it also 
binds to XPA, TFIIH, and the p34 subunit of TFIIE. These interactions are likely 
to play a role in recruiting repair proteins to ternary complexes formed at 
damage sites.