Fig. 5. (A) 5′-labeled 20 bp DNA was treated with exonuclease III in the presence of the indicated amounts of HU, phenol extracted and electrophoresed. (B) Circular DNA containing a nick was treated with exonuclease III in the presence of the indicated amounts of HU, phenol extracted and hybridized with 5′-labeled oligonucleotide of the sequence –75 to –50 (with respect to the position of the nick) of the discontinuous strand. The hybridization signal shows the amount of continuous DNA strand that is ss as a result of the degradation of the nicked strand. Hybridized DNA was separated from the labeled oligonucleotide by electrophoresis and quantified. (C) Graph of hybridization yield versus time of exonuclease treatment, and estimation of the time necessary so that half of the continuous strand was free to be hybridized with complementary oligonucleotide.