Ultraviolet light induced pyrimidine dimers in DNA are recognized and repaired 
by a number of unique cellular surveillance systems. At the highest level of 
complexity Escherichia coli (E. coli) has a uvr DNA repair system comprising the 
UvrA, UvrB and UvrC proteins responsible for incision. There are several 
preincision steps governed by this pathway which includes an ATP-dependent UvrA 
dimerization reaction required for UvrAB nucleoprotein formation. This complex 
formation driven by ATP binding, is associated with localized topological 
unwinding of DNA. This protein complex can catalyze an ATP-dependent 5'----3' 
directed strand displacement of D-loop DNA or short single strands annealed to a 
single stranded circular or linear DNA. This putative translocational process is 
arrested when damaged sites are encountered. The complex is now primed for dual 
incision catalyzed by UvrC. The remainder of the repair process involves UvrD 
(helicase II) and DNA polymerase I for a coordinately controlled "excision 
resynthesis" step accompanied by UvrABC turnover. Furthermore, it is proposed 
that levels of repair proteins can be regulated by proteolysis. UvrB is 
converted to truncated UvrB* by a stress induced protease which also acts at 
similar sites on the E. coli Ada protein. Although UvrB* can bind with UvrA to 
DNA it cannot participate in helicase or incision reactions. It is also a 
DNA-dependent ATPase.