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 recombination. We examined the joining of 3'-protruding single strand (PSS) ends, which do not have recessed 3' hydroxyls that can allow for fill-in DNA synthesis, to blunt ends. End-joining was examined by electro-transforming Escherichia coli strains with linearized plasmid DNA, sequencing the resulting junctions, and determining the transformation frequencies. Three different E.coli strains were examined: MC1061, which has no known recombination or DNA repair defects, HB101 (rec A-) and SURE (recB- recJ-). No striking differences were found in either the spectrum of products observed or the efficiency of end-joining between these strains. As in vertebrate systems, the majority of the products were overlaps between directly repeated DNA sequences. 3'-PSS are frequently preserved in vertebrate systems, but they were not preserved in our experiments unless the transforming DNA was pretreated with a DNA polymerase.