Regulation of gene transcription in both prokaryotes and eukaryotes involves the formation of DNA-multiprotein complexes. These complexes build a precise three-dimensional topology allowing communication between distal regions of DNA. The switch from early to late transcription in bacteriophage Ø29 involves binding of viral proteins, p4 and p6, to a region of the genome containing the early promoters A2c and A2b and the late promoter A3. Atomic force microscopy imaging under aqueous buffering conditions of complexes built after DNA incubation with proteins p4 and p6 shows the formation of a nucleoprotein arrangement with consistent morphology. These two low specificity DNA binding proteins are capable of bending 160 base pairs into a nucleoprotein-hairpin stable enough to be imaged by AFM. The functional implications of this nucleoprotein-hairpin in the coordinated regulation of early and late promoters are discussed.
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