SummaryWe report here that homologous recombination functions are required for the viability of Escherichia coli cells maintaining a 240 bp chromosomal inverted repeat (palindromic) sequence. Wild-type cells can successfully replicate this palindrome but recA, recB or recC mutants carrying the palindrome are unviable. The dependence on homologous recombination for cell viability is overcome in sbcC mutants. Directly repeated 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 sequence by the SbcCD nuclease are repaired by homologous recombination. The repair is conservative and the palindrome is retained in the repaired chromosome. We conclude that SbcCD can attack secondary structures but that repair conserves the DNA sequence with the potential to fold.
SummaryWe have isolated and sequenced a set of deletions stimulated by DNA palindromes in Escherichia coli. All of the deletions are asymmetric with respect to the parental sequence and have occurred at short direct repeats. This is consistent with deletion by strand slippage during DNA replication. The orientation of the asymmetry in such deletion products is diagnostic of the direction of the strand slippage event. It is therefore also diagnostic of its occurrence on the leading or lagging strand of the replication fork when the direction of replication is known. In all cases in which the orientation of the asymmetry could be determined with respect to DNA replication, the products were consistent with a preference for deletion on the lagging strand of the fork. The data include replication slippage in three situations: on the chromosome of E. coli, in bacteriophage and in high-copy-number pUC-based plasmids.
A novel DNA rearrangement has been characterised that is both a direct and inverted repeat. This rearrangement involves the 2-fold duplication of a plasmid sequence adjacent to the site of insertion of a long palindrome. The sequence of this rearrangement suggests that it has arisen by strand slippage from the leading to the lagging strand of the replication fork as a consequence of the presence of the long palindrome.
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