REPAIR BY RECOMBINATION OF DNA CONTAINING A PALINDROMIC SEQUENCE

We report here that homologous recombination functions ape required fo r the viability of Escherichia coli cells maintaining a 240 bp chromos omal inverted repeat (palindromic) sequence. Wild-type cells can succe ssfully replicate this palindrome but recA, recB or recC mutants carry ing the palindrome are unviable. The dependence on homologous recombin ation for cell viability is overcome in sbcC mutants. Directly repeate d copies of the DNA containing the palindrome are rapidly resolved to single copies in wild-type cells but not in sbcC mutants. Our results suggest that double-strand breaks introduced at the palindromic DNA se quence by the SbcCD nuclease are repaired by homologous recombination. The repair is conservative and the palindrome is retained in the repa ired chromosome. We conclude that SbcCD can attack secondary structure s but that repair conserves the DNA sequence with the potential to fol d.