Xf. Pan et Drf. Leach, The roles of mutS, sbcCD and recA in the propagation of TGG repeats in Escherichia coil, NUCL ACID R, 28(16), 2000, pp. 3178-3184
A 24 triplet TGG . CCA repeat array shows length- and orientation-dependent
propagation when present in the plasmid pUC18, When TGG(24) is present as
template for leading-strand synthesis, plasmid recovery is normal in all st
rains tested. However, when it acts as template for lagging-strand synthesi
s, plasmid propagation is seriously compromised. Plasmids carrying deletion
s in the 5' side of this sequence can be isolated and products carrying 15
TGG triplets do not significantly interfere with plasmid propagation. Mutat
ions in sbcCD, mutS and recA significantly improve the recovery of plasmids
with TGG(24) on the lagging-strand template. These findings suggest that T
GG(24) can fold into a structure that can interfere with DNA replication in
vivo but that TGG(15) 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 cleavag
e are implicated, In contrast, there is no correlation of triplet repeat ex
pansion 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.