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Genomic analysis of LexA binding reveals the permissive nature of the Escherichia coli genome and identifies unconventional target sites.

Genomes of eukaryotic organisms are packaged into nucleosomes that restrict the binding of transcription factors to accessible regions. Bacteria do not contain histones, but they have nucleoid-associated proteins that have been proposed to function analogously. Here, we combine chromatin immunoprecipitation and high-density oligonucleotide microarrays to define the in vivo DNA targets of the LexA transcriptional repressor in Escherichia coli. We demonstrate a near-universal relationship between the presence of a LexA sequence motif, LexA binding in vitro, and LexA binding in vivo, suggesting that a suitable recognition site for LexA is sufficient for binding in vivo. Consistent with this observation, LexA binds comparably to ectopic target sites introduced at various positions in the genome. We also identify approximately 20 novel LexA targets that lack a canonical LexA sequence motif, are not bound by LexA in vitro, and presumably require an additional factor for binding in vivo. Our results indicate that, unlike eukaryotic genomes, the E. coli genome is permissive to transcription factor binding. The permissive nature of the E. coli genome has important consequences for the nature of transcriptional regulatory proteins, biological specificity, and evolution.