Mutagenesis at [GT/CA]10 [TC/AG]11 and [TTCC/AAGG]9 microsatellite sequences inserted in the herpes simplex virus thymidine kinase (HSV-tk) gene was analyzed in isogenic mut(L)+ and mut(L)- Escherichia coli. In both strains, significantly more expansion than deletion mutations were observed at the [TTCC/AAGG]9 motif relative to either dinucleotide motif. As the HSV-tk coding sequence contains an endogenous [G/C]7 mononucleotide repeat and ~1000 bp of unique sequence, we were able to compare mutagenesis among various sequence motifs. We observed that the relative risk of mutation in E.coli is: [TTCC/AAGG]9 > [TTCC/AAGG]10 ~ [TC/AG]11 > unique ~ [G/C]7. The mutation frequency varied 1400- fold fold in mutL+ cells between the tetranucleotide motif and the mononucleotide motif, but only 50-fold in mutL- cells. The [G/C]7 sequence was destabilized the greatest and the tetranucleotide motif the least by loss of mismatch repair. These results demonstrate that the quantitative risk of mutation at various microsatellites greatly depends on the DNA sequence composition. We suggest alternative models for the production of expansion mutations during lagging strand replication of the [TTCC/AAGG]9 microsatellite.
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