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DNA methylation-dependent phase variation of the pap operon in E. coli. Proteins that are essential for the “on” and “off” phases, and the relative positions of their binding sites, are depicted. Note that the methylation state of the two GATC DNA sequences, depicted by Me, also differ in the two phases as a result of the accessibility of Dam. The drawing is not to scale and is not meant to convey protein size or biochemical properties. See the text for additional information.