(CAG)·(CTG) Repeats Associated with Neurodegenerative Diseases Are Stable in the Escherichia coli Chromosome

(CAG)n·(CTG)n expansion is associated with many neurodegenerative diseases. Repeat instability has been extensively studied in bacterial plasmids, where repeats undergo deletion at high rates. We report an assay for (CAG)n·(CTG)n deletion from the chloramphenicol acetyltransferase gene integrated into the Escherichia coli chromosome. In strain AB1157, deletion rates for 25–60 (CAG)·(CTG) repeats integrated in the chromosome ranged from 6.88 × 10–9 to 1.33 × 10–10, or ∼6,300 to 660,000-fold lower than in plasmid pBR325. In contrast to the situation in plasmids, deletions occur at a higher rate when (CTG)43, rather than (CAG)43, comprised the leading template strand, and complete rather than partial deletions were the predominant mutation observed. Repeats were also stable on long term growth following multiple passages through exponential and stationary phase. Mutations in priA and recG increased or decreased deletion rates, but repeats were still greatly stabilized in the chromosome. The remarkable stability of (CAG)n·(CTG)n repeats in the E. coli chromosome may result from the differences in the mechanisms for replication or the probability for recombination afforded by a high plasmid copy number. The integration of (CAG)n·(CTG)n repeats into the chromosome provides a model system in which the inherent stability of these repeats reflects that in the human genome more closely.