Role of proofreading and mismatch repair in maintaining the stability of nucleotide repeats in DNA

The role of the proofreading exonuclease in maintaining the stability of multiply repeated units in DNA was studied in Escherichia coli. Reversion of plasmids in which the β-galactosidase α complementing sequence was moved +2 out of frame by inserts containing (CA)14, (CA)5, (CA)2 or (TA)6 or +1 by creating a run of 8 C was compared in mutS and mutSdnaQ strains. Proofreading corrects at least half of the frameshift errors for all the plasmids and at least 99% of the errors in the (CA)2 plasmid. The (CA)2 plasmid reverts mostly by +1 frameshifts in the restriction sites flanking the insert. With the (CA)14, (TA)6, (CA)5 and 8C plasmids, reversion is mainly by loss of a repeat unit. The data support the hypothesis that the dnaQ gene product recognizes frameshifts close to the DNA growing point. Frameshifts distal to the growing point are mainly corrected by mismatch repair. We speculate that mismatches in mononucleotide repeats are susceptible to proofreading because they can either migrate to a point where they are recognized by the exonuclease or, alternatively, because single nucleotide distortions are more readily detected than dinucleotides.