Mismatch repair at stop codons is directed independent of GATC methylation in Escherichia coli
Szabolcs Semsey, Center for Models of Life, Niels Bohr Institute, Copenhagen
The mismatch repair system (MMR) corrects replication errors that escape proofreading. Previous studies in Escherichia coli suggested that MMR uses hemimethylated GATC sites to identify the newly synthesized strand. We addressed the effect of local GATC content on the rate of reversion of a stop codon using an engineered reporter gene placed on the E. coli chromosome. We show that occurrence of point mutations at the stop codon is not affected by the distance to the next GATC site but can be influenced by changing the sequence context 50 base pairs away. The effect of the chromosomal context is not eliminated by deletion of dam, mutL or mutS. Surprisingly, the reversion rate in the dam mutant increased much less than in the mutL and mutS mutants. In this paper we argue that similar to other organisms, GATC methylation may not be the major strand discrimination signal in E. coli MMR.