UNG directs repair of only a fraction of U•G mispairs in human nuclear extracts. (A) Products of deglycosylation of a 5'-end-labeled single-stranded oligonucleotide containing a single uracil, following incubation with recombinant UDG or nuclear extracts prepared from HeLa, Ramos, and LoVo cell lines in the presence (+) or absence (-) of Ugi. Substrates were treated with alkali following incubation to hydrolyze the backbone at abasic sites. Arrows indicate substrate and product. (B) Representative reactions demonstrating repair of M13-U•G substrates by nuclear extracts of HeLa or Ramos, in the presence (+) or absence (-) of Ugi, analyzed by agarose gel electrophoresis. Arrows at left indicate unrepaired and repaired products; fragment sizes are shown at right; fraction of molecules repaired is shown below each lane.