A model is described whereby (i) regulatory proteins recognize one face of the DNA double helix on non-adjacent DNA regions brought close together in space through folding around nucleosomes, (if) regulatory sequences may occur inside gene structures, and (iii) the recognition of regulatory sequences might be modulated by short (4–6 base-pairs) insertions or deletions in introns or upstream from the transcription start site. This model might apply not only to eucaryotes but also to procaryotic organisms whose DNA is organized through interactions with histone-like proteins. Consequences of the model regarding the binding of regulatory proteins in procaryotes are suggested.

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