Near-continuously synthesized leading strands in Escherichia coli are broken by ribonucleotide excision

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In vitro, purified replisomes drive model replication forks to synthesize continuous leading strands, even without ligase, supporting the semidiscontinuous model of DNA replication. However, nascent replication intermediates isolated from ligase-deficient Escherichia coli comprise only short (on average 1.2-kb) Okazaki fragments. It was long suspected that cells replicate their chromosomal DNA by the semidiscontinuous mode observed in vitro but that, in vivo, the nascent leading strand was artifactually fragmented postsynthesis by excision repair. Here, using high-resolution separation of pulse-labeled replication intermediates coupled with strand-specific hybridization, we show that excision-proficient E. coli generates leading-strand intermediates >10-fold longer than lagging-strand Okazaki fragments. Inactivation of DNA-repair activities, including ribonucleotide excision, further increased nascent leading-strand size to ∼80 kb, while lagging-strand Okazaki fragments remained unaffected. We conclude that in vivo, repriming occurs ∼70× less frequently on the leading versus lagging strands, and that DNA replication in E. coli is effectively semidiscontinuous.

Original languageEnglish (US)
Pages (from-to)1251-1260
Number of pages10
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number4
StatePublished - Jan 22 2019


  • Ligase mutant
  • Okazaki fragments
  • Replication intermediates
  • Ribonucleotide excision repair
  • The leading strand

ASJC Scopus subject areas

  • General


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