Two distinct models account for short and long deletions within sequence repeats in Escherichia coli

In Escherichia coli, (GpC)n sequences cloned into plasmid DNA molecules are deletion-prone with the occurrence of both short (<2 bp) and long (>2 bp) deletion events. These repetitive tracts can be stabilized by interrupting the strict monotony of the repetition with a variant dinucleotide sequence. The stabilization of short deletion events that is mediated by the variant sequence is completely lost in E. coli mismatch repair-deficient strains. In contrast, this repair pathway has no influence on the frequency of occurrence of long deletion events, even in sequences containing the variant repeat. These results lead us to propose two distinct models to account for short and long deletions within repetitive sequences in E. coli. Furthermore, this study reveals that the deletions occur preferentially at the end of the repeat sequence that is distal with respect to the origin of replication.