AVIAN NUCLEAR MATRIX PROTEINS BIND VERY TIGHTLY TO CELLULAR DNA OF THE BETA-GLOBIN GENE ENHANCER IN A TISSUE-SPECIFIC FASHION

We have previously shown that a cloned 480 bp DNA fragment that spans the 3'-enhancer region of the avian beta-globin gene cluster can become very tightly, perhaps covalently, bound to protein in avian nuclear matrices in vitro [Zenk et al. (1990) Biochemistry 29, 5221-5226]. This binding was not tissue-specific and was probably not mediated by topoisomerase enzymes. In the present study, we have examined avian nuclear matrices (or scaffolds) for the presence of very tight cellular DNA-protein complexes in the region of the beta-globin gene enhancer and of several other avian genes. Nuclear matrices were prepared by both high- and low-salt methods, and protein - DNA complexes were isolated by SDS/K+ precipitation after restriction enzyme digestion. In adult reticulocytes, up to 30% of the intact 3800 bp HindIII - EcoRI fragment that encompasses the beta-globin enhancer element may be very tightly bound to nuclear matrix protein. In adult avian thymus nuclei, the beta-globin enhancer is neither matrix-associated nor tightly bound to protein. In contrast, a 5.0-kb HindIII fragment of the malic enzyme gene is very tightly bound to nuclear matrix-associated protein in thymus cells, but not reticulocytes. The malic enzyme gene is active in thymus cells, and not in reticulocytes. These results suggest that certain regions of avian cellular DNA are very tightly, perhaps covalently, attached to nuclear matrix-associated proteins. Attachment follows a tissue-specific pattern that is associated with transcriptional activity.