Characterization of a multisubunit human protein which selectively binds single stranded d(GA)nand d(GT)nsequence repeats in DNA

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Abstract
A protein which selectively binds d(GA)n and d(GT)n sequence repeats in single stranded DNA has been identified in human fibroblasts. This protein, designated PGB, has been purified at least 500-fold by ammonium sulfate precipitation followed by DEAE-Sepharose column chromatography and affinity chromatography in a column of d(GA)-Sepharose. Electrophoretic mobility shift assays revealed that the PGB protein bound most avidly d(GA)n and d(GT)n tracts of n > 5. It also bound other G-rich DNA sequence repeats, including dGn tracts, with lower affinities. It did not manifest significant binding affinities to single stranded M13 DNA, or to the homopolynucleotides poly dA, poly dC and poly dT, or to various DNA sequence repeats which do not contain G residues, such as d(A-C)n and d(TC)n. It did not bind double stranded d(T-C)n.d(GA)n tracts or other double stranded DNA sequences. In glycerol gradient centrifugation assays the d(GA)n- and the d(GT)n-binding activities cosedimented as a homogeneous protein species having an S20,w = 9.4 +/- 0.7 and an estimated native molecular weight of 190,000 +/- 7,000. UV crosslinking assays revealed that the protein contains 33.6 +/- 2.1 kd subunits which bind d(GA)n and d(GT)n sequences. However, SDS-polyacrylamide gel electrophoresis of the purified protein followed by silver staining indicated that it may also contain other subunits that do not contact the DNA. It is proposed that binding of the PGB protein to single stranded d(GA)n or d(GT)n tracts in double stranded topologically restricted DNA may stimulate strand separation and formation of triple helices or other unusual DNA structures.

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