Nucleotide excision repair affects the stability of long transcribed (CTG center dot CAG) tracts in an orientation-dependent manner in Escherichia coli

The influence of nucleotide excision repair (NER), the principal in vivo re pair system for DNA damages, was investigated in Escherichia coil with uvrA , uvrB and uvrAuvrB mutants with the triplet repeat sequences (TRS) involve d in myotonic dystrophy, the fragile X syndrome and Friedreich's ataxia, (C TG.CAG)(175) was more stable when the (CTG) strand was transcribed than whe n the (CAG) strand was transcribed in the alternate orientation. A lack of the UvrA protein dramatically increases the instability of this TRS in vivo as compared with the stability of the same sequence in uvrB mutant, which produces an intact UvrA protein. We propose that transcription transiently dissociates the triplet repeat complementary strands enabling the nontransc ribed strand to fold into a hairpin conformation which is then sufficiently stable that replication bypasses the hairpin to give large deletions. If t he TRS was not transcribed, fewer deletions were observed, Alternatively, i n the uvrA(-) mutant, the hairpins existing on the lagging strand will suff er bypass DNA synthesis to generate deleted molecules, Hence, NER, function ally similar in both prokaryotes and eukaryotes, is an important factor in the genetic instabilities of long transcribed TRS implicated in human hered itary neurological diseases.