Protein-DNA complexes isolated in gel retardation assays can be digested within the acrylamide matrix by the nuclease activity of 1,10-phenanthroline-copper ion (OP-Cu). When the oligonucleotide products are eluted and analyzed on a sequencing gel, a footprint of the DNA-protein complex is obtained. Therefore, any protein-DNA complex isolated by the widely used gel retardation technique can be defined in terms of sequence-specific interactions by this simple methodology. The binding of the lac repressor and Escherichia coli RNA polymerase to an EcoRI fragment containing the lac control region has been studied by the combined gel retardation-1,10-phenanthroline-copper ion footprinting procedure. Footprints of lac repressor binding correspond to those obtained in solution with OP-Cu and DNase I and verify the experimental procedures. In studying E. coli RNA polymerase-promoter complexes, we have found that magnesium ion is required to form single-stranded DNA structures characteristic of kinetically competent open transcription complexes.

The self-complementing dodecamer 5'-CGCGAATTCGCG-3' and its complexes with the antibiotic netropsin and the restriction endonuclease EcoRI provide substrates of known three-dimensional structure to study the stereochemistry and mechanism of the artificial nuclease of 1,10-phenanthroline-copper ion [(OP)2Cu+]. Analysis of the reaction products with the 5'-32P dodecamer on 20% sequencing gels has demonstrated the presence of 3'-phosphoglycolate ends in addition to 3'-phosphomonoester ends expected from previous studies. A reaction intermediate, which is a precursor to 3'-phosphomonoester termini, has been trapped; in contrast, no comparable species for the 5'-phosphomonoester termini can be detected when 3'-labeled DNAs are utilized as substrates. The reactive oxidative species formed by the coreactants (OP)2Cu+ and hydrogen peroxide is distinguishable in its chemistry from the hydroxyl radicals produced by cobalt-60 gamma-irradiation. The freely diffusible hydroxyl radicals generated by cobalt-60 irradiation produce equivalent amounts of 3'-phosphomonoester and 3'-phosphoglycolate termini whereas the 3'-phosphomonoesters are the preferred product of (OP)2Cu+ and H2O2. On the basis of the structures of the products obtained, the principal site of attack of the coordination complex is on the C-1 of the deoxyribose within the minor groove. This conclusion is supported by the footprinting of netropsin binding to the dodecamer. Crystallographic results have demonstrated that netropsin binds to the minor groove at the central AATT residue. A clear protection of attack by the coordination complex at the deoxyriboses associated with A-5, T-6, T-7, and C-9 is fully consistent with attack from the minor groove without intercalation during the course of the cleavage reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

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The transactivator protein, tat, encoded by the human immunodeficiency virus is a key regulator of viral transcription. Activation by the tat protein requires sequences downstream of the transcription initiation site called the transactivating region (TAR). RNA derived from the TAR is capable of forming a stable stem-oop structure and the maintenance of both the stem structure and the loop sequences located between + 19 and +44 is required for complete in vivo activation by tat.Gel retardation assays with RNA from both wild-type and mutant TAR constructs generated in viro with SP6 polymerase indicated specific binding ofHeLa nuclear proteins to the TAR. To characterize this RNA-protein interaction, a method of chemical "imprinting" has been developed using photoactivated uranyl acetate as the nucleolytic agent. This reagent nicks RNA under physiological conditions at all four nucleotides in a reaction that is independent of sequence and secondary structure. Specific interaction of cellular proteins with TAR RNA could be detected by enhanced cleavages or imprints surrounding the loop region. Mutations that either disrupted stem base-pairing or extensively changed the primary sequence resulted in alterations in the cleavage pattern of the TAR RNA. Structural features of the TAR RNA stem-oop essential for tat activation are also required for specific binding of the HeLa cell nuclear protein.