THE JOINING OF BLUNT DNA ENDS TO 3'-PROTRUDING SINGLE STRANDS IN ESCHERICHIA-COLI

In eukaryotic and prokaryotic organisms DNA double-strand breaks with non-complementary ends can be joined by mechanisms of illegitimate rec ombination. We examined the joining of 3'-protruding single strand (PS S) ends, which do not have recessed 3' hydroxyls that can allow for fi ll-in DNA synthesis, to blunt ends. End-joining was examined by electr o-transforming Escherichia coli strains with linearized plasmid DNA, s equencing the resulting junctions, and determining the transformation frequencies. Three different E.coli strains were examined: MC1061, whi ch has no known recombination or DNA repair defects, HB101 (recA(-)) a nd SURE (recB(-) recJ(-)). No striking differences were found in eithe r the spectrum of products observed or the efficiency of end-joining b etween these strains. As in vertebrate systems, the majority of the pr oducts were overlaps between directly repeated DNA sequences. 3'-PSS a re frequently preserved in vertebrate systems, but they were not prese rved in our experiments unless the transforming DNA was pretreated wit h a DNA polymerase.