Overexpression of the single-stranded DNA-binding protein (SSB) stabilises CAG*CTG triplet repeats in an orientation dependent manner.

The stability and deletion-size-distribution profiles of leading strand (CAG)(75) and (CTG)(137) trinucleotide repeat arrays inserted in the Escherichia coli chromosome were investigated upon overexpression of the single-stranded DNA-binding protein (SSB) and in mutant strains deficient for the SbcCD (Rad51/Mre11) nuclease. SSB overexpression increases the stability of the (CAG)(75) repeat array and leads to a loss of the bias towards large deletions for the same array. Furthermore, the absence of SbcCD leads to a reduction in the number of large deletions in strains containing the (CTG)(137) repeat array.