Organization of the 3′-boundary of the chicken α globin gene domain and characterization of a CR 1-specific protein binding site

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Abstract
The sequence of a DNA fragment about 1 Kbp long located at the 3′ boundary of the chicken α globin gene domain, including the 3′-side matrix attachment point and the site of transcription termination, was determined. It contains a repetitive DNA element and the AT-rich (easily denaturable) DNA segment conserved at the same position in the duck genome. The repetitive sequence was identified by computer analyzis as being a member of the CR1 family. Within the non-repetitive part of the AT-rich DNA fragment, four topoisomerase II recognition sites were found which might be indicative of matrix attachment. Furthermore, two distinct regions were identified, possessing strong homology to a number of non-coding consensus sequences, one of them to a limited part of the LTR of HTLVIII, and the other to the replication origin of Polyoma virus JC. DNA shift experiments showed that the CR1 repeat binds specifically an abundant nuclear protein factor. The binding site for this factor was identified by footprinting and turned out to be closely related to the previously described recognition site for the TGGCA-binding protein, the chicken analog of nuclear factor 1 (NF-1). Finally, the CR1 repeats within the chicken α and β globin gene domains were mapped. All these observations are discussed in terms of the organization of the 5′ and 3′ boundaries of the functional genomic domains forming a chromatin loop including all avian α type globin genes.