Nuclear protein which selectively binds to the Alu-family DNA repeat (AFR, Blur8) is partially purified from human HeLa cells using a gel retention assay. At low protein concentrations only a single complex of the protein with AFR is formed (CII). Increasing protein concentrations lead to the gradual disappearance of CII, being replaced by complexes with higher (CI) and lower (CIII, CIV) electrophoretic mobilities. Differential binding of AFR restriction subfragments indicates that multiple protein-binding sites are present within AFR. We discuss two models explaining the anomalous electrophoretic mobility of CII by DNA bending or looping upon cooperative multi-site binding of the protein to AFR.
