The binding of a steroid receptor to specific nuclear sites (i.e., nuclear acceptor sites) represents the immediate event preceding the steroid regulation of gene transcription. How the same steroid receptor regulates different genes in different tissues is unknown. Since a major fraction of the nuclear acceptor sites for a variety of steroid receptors has been reported to be masked in the chromatins of a variety of tissues, the differential expression of the nuclear acceptor sites may explain this regulation of different genes. In the avian oviduct, the removal of a subfraction of chromosomal non-histone proteins, termed CP-2, results in the unmasking of the nuclear acceptor sites for the progesterone receptor (PR). Further, the extent of masking of these nuclear acceptor sites for PR has been reported to vary during cytodifferentiation of the avian oviduct. This paper describes a method for the reconstitution of the masking of PR nuclear acceptor sites in the avian oviduct chromatin using a partially purified chromosomal protein fraction (CP-2b). The reannealling of the CP-2b fraction to unmasked avian oviduct chromatin (termed nucleoacidic protein or NAP) results in the "remasking" of about the same number of nuclear acceptor sites for PR as found in intact chromatin. Because some of the PR acceptor sites on the NAP cannot be remasked, these sites either must be protected from masking or not be recognized by the masking proteins. The masking activity apparently involves only protein(s) because the unmasking of acceptor sites can be achieved with protease but not ribonuclease activities and because the dissociated masking activity is destroyed only by proteases. The masking appears to be reversible because the reconstituted masked sites can again be unmasked. Preliminary purification and characterization of the masking activity in fraction CP-2b by molecular sieve chromatography indicate a heterogeneity of size with the activity eluting in a molecular weight range of from 60000 to > 150 000. Whether the masking proteins prevent the binding of the progesterone receptor by directly binding the acceptor sites or by binding neighboring domains to condense the chromatin is unknown. It is speculated that the masking of acceptor sites may be responsible in part for determining the tissue-specific gene expression induced by steroids and/or may play a role in the unresponsiveness of certain human tumors containing steroid receptors.
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