STRUCTURAL CHARACTERIZATION OF SP1-DNA COMPLEXES

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Synthesis of messenger RNA by RNA polymerase II requires the interaction of a large array of auxiliary transcription factors that recognize and bind to specific promoter DNA sequences located upstream of eukaryotic genes. These transcription factors regulate the initiation of transcription in a temporally ordered manner by assembling and engaging the active transcription complex. In order to understand the detailed roles played by transcription factors, efforts have been made to fractionate the factors necessary to reconstitute transcriptional activity in vitro. These experiments have resulted in the identification, purification and characterization of one such promoter-specific transcription factor, Sp l, from HeLa cells. Sp l enhances transcription from a variety of viral and cellular genes by binding to one or more "GC box" recognition elements (containing a hexanuclear core GGGCGG) within the 5' flanking promoter sequences through the use of three "zinc-finger" domains. In general, DNA binding surfaces are designed to have a highly defined preference for their cognate DNA binding site; Sp l is unique among transcription factors identified to date in that it recognizes a host of transcription activating binding sites that can be classified as either high, medium or low affinity. We are therefore quite interested in examining the binding of Sp1 to "GC box" sequences in order to define those factors responsible for this unusually promiscuous sequence recognition ability. The Spl system is amenable to detailed examination owing to localization of its DNA binding properties to the "zinc-finger" domain. This proposal describes an approach that will enable us to exploit the relatively compact "zinc-finger" motif to define those structural factors responsible for the recognition diversity of Sp 1. Our objectives are: ( 1) to express short soluble Sp 1 fragments that contain the three "zinc finger" domains, (2) to test the ability of these fragments to duplicate the natural binding properties of intact Spl by band shift assays, (3) to express and purify large quantities of those peptides that exhibit Spl binding ability, (4) to quantitatively characterize the interactions of these fragments with duplex oligonucleotides by molecular biological (band shift assays, competition assays, mutagenesis studies) and biophysical techniques (calorimetry, NMR) in order to define the chemical/structural basis for the observed diversity in DNA binding, and (5) to determine the solution structures of the active Spl fragments by NMR spectroscopic techniques. Our long term objective is to characterize the solution structures of a number of Spl -DNA complexes using a series of high and medium affinity DNA recognition sequences. Such data will not only contribute to our understanding at the molecular level of how "zinc-finger" domains are used in molecular recognition but also how this major structural motif is able to recognize a highly variable set of DNA sequences.