2 DISTINCT MODELS ACCOUNT FOR SHORT AND LONG DELETIONS WITHIN SEQUENCE REPEATS IN ESCHERICHIA-COLI

In Escherichia coli, (GpC)(n) sequences cloned into plasmid DNA molecu les are deletion-prone with the occurrence of both short (<2 bp) and l ong (>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 mis match repair-deficient strains. In contrast, this repair pathway has n o influence on the frequency of occurrence of long deletion events, ev en in sequences containing the variant repeat. These results lead us t o propose two distinct models to account for short and long deletions within repetitive sequences in E. coli. Furthermore, this study reveal s that the deletions occur preferentially at the end of the repeat seq uence that is distal with respect to the origin of replication.