The roles of mutS, sbcCD and recA in the propagation of TGG repeats in Escherichia coli

A 24 triplet TGG·CCA repeat array shows length- and orientation-dependent propagation when present in the plasmid pUC18. When TGG₂₄ is present as template for leading-strand synthesis, plasmid recovery is normal in all strains tested. However, when it acts as template for lagging-strand synthesis, plasmid propagation is seriously compromised. Plasmids carrying deletions in the 5' side of this sequence can be isolated and products carrying 15 TGG triplets do not significantly interfere with plasmid propagation. Mutations in sbcCD, mutS and recA significantly improve the recovery of plasmids with TGG₂₄ on the lagging-strand template. These findings suggest that TGG₂₄ can fold into a structure that can interfere with DNA replication in vivo but that TGG₁₅ cannot. Furthermore, since the presence of the MutS and SbcCD proteins are required for propagation interference, it is likely that stabilisation of mismatched base pairs and secondary structure cleavage are implicated. In contrast, there is no correlation of triplet repeat expansion and deletion instability with predicted DNA folding. These results argue for a dissociation of the factors affecting DNA fragility from those affecting trinucleotide repeat expansion-contraction instability.