Mechanism of promoter repression by Lac repressor-DNA loops

Nicole A. Becker, Justin P. Peters, L. James Maher

Research output: Contribution to journalArticlepeer-review

29 Scopus citations


The Escherichia coli lactose (lac) operon encodes the first genetic switch to be discovered, and lac remains a paradigm for studying negative and positive control of gene expression. Negative control is believed to involve competition of RNA polymerase and Lac repressor for overlapping binding sites. Contributions to the local Lac repressor concentration come from free repressor and repressor delivered to the operator from remote auxiliary operators by DNA looping. Long-standing questions persist concerning the actual role of DNA looping in the mechanism of promoter repression. Here, we use experiments in living bacteria to resolve four of these questions. We show that the distance dependence of repression enhancement is comparable for upstream and downstream auxiliary operators, confirming the hypothesis that repressor concentration increase is the principal mechanism of repression loops. We find that as few as four turns of DNA can be constrained in a stable loop by Lac repressor. We show that RNA polymerase is not trapped at repressed promoters. Finally, we show that constraining a promoter in a tight DNA loop is sufficient for repression even when promoter and operator do not overlap.

Original languageEnglish (US)
Pages (from-to)156-166
Number of pages11
JournalNucleic acids research
Issue number1
StatePublished - Jan 2013

ASJC Scopus subject areas

  • Genetics


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