Developmental regulation of the aldolase A muscle-specific promoter during in vivo muscle maturation is controlled by a nuclear receptor binding element.

During the post-natal period, skeletal muscles undergo important modifications leading to the appearance of different types of myofibers which exhibit distinct contractile and metabolic properties. This maturation process results from the activation of the expression of different sets of contractile proteins and metabolic enzymes, which are specific to the different types of myofibers. The muscle-specific promoter of the aldolase A gene (pM) is expressed mainly in fast-twitch glycolytic fibers in adult body muscles. We investigate here how pM is regulated during the post-natal development of different types of skeletal muscles (slow or fast-twitch muscles, head or body muscles). We show that pM is expressed preferentially in prospective fast-twitch muscles soon after birth; pM is up-regulated specifically in body muscles only later in development. This activation pattern is mimicked by a transgene which comprises only the 355 most proximal sequences of pM. Within this region, we identify a DNA element which is required for the up-regulation of the transgene during post-natal development in body muscles. Comparison of nuclear M1-binding proteins from young or adult body muscles show no qualitative differences. Distinct M1-binding proteins are present in both young and adult tongue nuclear extracts, compared to that present in gastrocnemius extracts.     

Interactions of the nuclear matrix-associated steroid receptor binding factor with its DNA binding element in the c-myc gene promoter

Steroid receptor binding factor (RBF) was originally isolated from avian oviduct nuclear matrix. When bound to avian genomic DNA, RBF generates saturable high-affinity binding sites for the avian progesterone receptor (PR). Recent studies have shown that RBF binds to a 54 bp element in the 5'-flanking region of the progesterone-regulated avian c-myc gene, and nuclear matrix-like attachment sites flank the RBF element [Lauber et al. (1997) J. Biol. Chem. 272, 24657-24665]. In this paper, electrophoretic mobility shift assays (EMSAs) and S1 nuclease treatment are used to demonstrate that the RBF-maltose binding protei (MBP) fusion protein binds to single-stranded DNA of its element. Only the N-terminal domain of RBF binds the RBF DNA element as demonstrated by southwestern blot analyses, and by competition EMSAs between RBF-MBP and the N-terminal domain. Mass spectrometric analysis of the C-terminal domain of RBF demonstrates its potential to form noncovalent protein-protein interactions via a potential leucine-isoleucine zipperlike structure, suggesting a homo- and/or possible heterodimer structure in solution. These data support that the nuclear matrix binding site (acceptor site) for PR in the c-myc gene promoter is composed of RBF dimers bound to a specific single-stranded DNA element. The dimers of RBF are generated by C-terminal leucine zipper and the DNA binding occurs at the N-terminal parallel beta-sheet DNA binding motif. This complex is flanked by nuclear matrix attachment sites.