CTG repeats show bimodal amplification in E. coli.

Trinucleotide repeats in human genetic disorders showing anticipation follow two inheritance patterns as a function of length. Inheritance of 35-50 repeats show incremental changes, while tracts greater than 80 repeats show large saltatory expansions. We describe a bacterial system that recapitulates this striking bimodal pattern of CTG amplification. Incremental expansions predominate in CTG tracts < Okazaki fragment size, while saltatory expansions increase in repeat tracts > or = Okazaki fragment size. CTG amplification requires loss of SbcC, a protein that modulates cleavage of single-stranded DNA and degradation of duplex DNA from double-strand breaks. These results suggest that noncanonical single strand-containing secondary structures in Okazaki fragments and/or double-strand breaks in repeat tracts are intermediates in CTG amplification.