The receptor-binding factor (RBF) for the avian oviduct progesterone (Pg) receptor (PR) has previously been shown to be a unique 10-kDa nuclear matrix protein that generates high affinity PR-binding sites on avian DNA. This paper describes the use of Southwestern blot and DNA gel shift analyses with RBF protein to identify a minimal 54-base pair RBF-binding element in the matrix-associated region (MAR) of the Pg-regulated c-myc gene promoter. This element contains a 5 -GC-rich domain and a 3 -AT-rich domain, the latter of which has a homopurine/homopyrimidine structure. The gel shift assays required the generation of an RBF-maltose fusion protein (RBF-MBP), which specifically binds this element and is supershifted when the anti-RBF polyclonal antibody is added. Computer analysis of the fulllength amino acid sequence for RBF predicts a DNAbinding motif involving a -sheet structure at the N-terminal domain. Southern blot analyses using nuclear matrix DNA suggests that there are dual MAR sites in the c-myc promoter, which flank an intervening domain containing the RBF element. The co-transfection of this MAR sequence, containing the RBF element and cloned into a luciferase reporter vector, together with an RBF expression vector construct, into steroid treated human MCF-7 cells, results in a decrease of the c-myc promoter activity relative to control transfections containing only the parent vector of the RBF expression construct. These data suggest that a unique chromatin/ nuclear matrix structure, composed of the RBF-DNA element complex which is flanked by nuclear matrix attachment sites, serves to bind the PR and repress the c-myc promoter.The classic model of steroid hormone action suggests that ligand-bound receptors dimerize and bind to specific sites (acceptor sites) on the chromatin/DNA to alter gene expression. In many genes, the steroid receptors (SR) 1 bind to specific cisacting DNA sequences, referred to as hormone response elements (for reviews, see Refs. 1-3). These elements function as enhancer-like DNA sequences and are required for the steroid hormone regulation of transcription. However, not all steroidresponsive genes contain these elements, and several other pathways and participating nuclear proteins have now been identified. The SR regulation of gene transcription has now been shown to involve 1) interactions of the SR with transcription factors, such as the AP-1 complex (for review, see Refs. 2-4); 2) interactions of SR with accessory factors, which appear to function by assisting the binding of the receptor to the steroid response elements, or to transcription factors (for reviews see Refs. 4 -6 and references therein); and finally, 3) interactions of SR with specific chromatin structures including the nuclear matrix (see Refs. 7-15 and references therein). In the latter, a class of high affinity binding sites (acceptor sites) for SRs in the nuclei, chromatin, and nuclear matrix DNAbinding proteins (e.g. acceptor proteins) has been reported by many laboratories for most of the steroid hormone...