Isolation of a protein fraction that binds preferentially to chicken middle repetitive DNA

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Abstract
Oviduct tissue extracts were fractionated by using a combination of ion-exchange and DNA-Sephadex chromatography. By comparing the electrophoretic patterns of proteins eluted from competing specific and nonspecific DNA columns, a fraction was isolated which bound with specificity to columns containing the chicken middle repetitive sequence CR1. This fraction showed a clear preference for binding to separate, cloned CR1 fragments derived from either the 5'' or the 3'' transition region of the ovalbumin gene domain when examined by using nitrocellulose filter binding assays. To localize the protein binding site, a CR1 clone was digested with various restriction enzymes, and the resulting fragments were examined for preferential protein binding. Results suggest that the binding site lies within a 39-nucleotide sequence which is highly conserved among different CR1 elements. This finding represents the first isolation of a protein which demonstrates a preference for binding to a middle repetitive sequence and suggests that this interaction may have a biological role. The DNA column competition adsorption method should have general application to the isolation of other gene-regulating proteins possessing DNA sequence preference.