Identification and analysis of a matrix‐attachment region 5′ of the rat glutamate‐dehydrogenase‐encoding gene

Eukaryotic chromatin is thought to be organized into independently regulated loop domains by interaction of matrix‐attachment regions (MAR) of the DNA to the nuclear matrix. To define the borders of the chromatin loop containing the glutamate dehydrogenase (GDH) gene, we screened the GDH gene and flanking regions for the presence of MAR sequences. We here report identification, mapping and sequencing of an (A+T)‐rich MAR located 2010–1397 bp upstream of the transcription initiation site of GDH, that mediates strong binding to the nuclear matrix. Smaller regions can also confer binding capacity, although at a lower affinity. This (A+T)‐rich MAR contained 11 bp and 12 bp (A+T)‐rich direct repeats, but not any of the sequences previously described to be associated with MAR activity. We here show that the presence of (A+T)‐rich domains of DNA is not sufficient to confer binding capacity, since (A+T)‐rich sequences located downstream of the identified MAR did not bind to the nuclear matrix. Moreover, a consensus topoisomerase‐II‐binding site located downstream of the MAR was found to be insufficient to mediate substantial binding. The number of binding sites in the nuclear matrix for MAR‐containing fragments was shown to be approximately 15 000/nucleus. Since organization of the entire rat genome in loops with an average loop size of 100 kbp would require 60 000 binding sites, this suggests that only part of the genome is organized in loops. Alternatively, we might have underestimated the number of binding sites. The GDH MAR, and MAR‐containing fragments derived from other species, were found to bind to the same binding sites in the nuclear matrix, although the affinity varied.