The myosin light chain enhancer and the skeletal actin promoter share a binding site for factors involved in muscle-specific gene expression.

The myosin light chain (MLC) 1/3 enhancer (MLC enhancer), identified at the 3' end of the skeletal MLC1/3 locus, contains a sequence motif that is homologous to a protein-binding site of the skeletal muscle alpha-actin promoter. Gel shift, competition, and footprint assays demonstrated that a CArG motif in the MLC enhancer binds the proteins MAPF1 and MAPF2, previously identified as factors interacting with the muscle regulatory element of the skeletal alpha-actin promoter. Transient transfection assays with constructs containing the chloramphenicol acetyltransferase reporter gene demonstrated that a 115-bp subfragment of the MLC enhancer is able to exert promoter activity when provided with a silent nonmuscle TATA box. A point mutation at the MAPF1/2-binding site interferes with factor binding and abolishes the promoter activity of the 115-bp fragment. The observation that an oligonucleotide encompassing the MAPF1/2 site of the MLC enhancer alone cannot serve as a promoter element suggests that additional factor-binding sites are necessary for this function. The finding that MAPF1 and MAPF2 recognize similar sequence motifs in two muscle genes, simultaneously activated during muscle differentiation, implies that these factors may have a role in coordinating the activation of contractile protein gene expression during myogenesis.     

The cell-specific enhancer of the mouse transthyretin (prealbumin) gene binds a common factor at one site and a liver-specific factor(s) at two other sites.

We previously defined two distinct cell-specific DNA elements controlling the transient expression of the transthyretin gene in Hep G2 (human hepatoma) cells: a proximal promoter region (-202 base pairs [bp] to the cap site), and a far-upstream cell-specific enhancer located between 1.6 and 2.15 kilobases (kb) 5' of the cap site (R. H. Costa, E. Lai, and J. E. Darnell, Jr., Mol. Cell. Biol. 6:4697-4708, 1986). In this report, we located the effective transthyretin enhancer element within a 100-bp region between 1.96 and 1.86 kb 5' to the mRNA cap site. In Hep G2 nuclear extracts, three protein-binding sites within this minimal enhancer element were identified by gel mobility and methylation protection experiments. Each binding site was required for full enhancer activity in Hep G2 transient expression assays. Competition experiments in protein-binding assays suggested that two of the three sites were recognized by a similar factor and that the protein interaction with the third site was different. The nuclear protein(s) which bound to the two homologous sites was found mainly or only in cells of hepatic origin, suggesting an involvement of this region in the cell-specific function of this enhancer. The nuclear protein(s) recognizing the third enhancer region was also found in HeLa and spleen cells.     

Functional analysis of the promoter of a sea urchin metallothionein gene.

The 5'-flanking region of the metallothionein (MT) gene LpMT1 of the sea urchin Lytechinus pictus includes three copies of a conserved sequence that includes the metal-responsive element (MRE) consensus core sequence required for heavy metal induction of other MT genes, a GC box, a G box of a putative basal level enhancer element which includes another MRE core element, and a poly(C) tract. A fragment of LpMT1 DNA from nucleotides +31 to -309 fused to a chloramphenicol acetyltransferase reporter gene was inducible with cadmium after injection into L. pictus embryos. This induced activity was greatly reduced in a deletion mutant which retained only 195 base pairs of 5'-flanking sequence, including the proximal pair of MREs and the G box, but excluding the poly(C) tract, GC box, and distal MRE. A potent human hMT-IIA gene promoter is marginally functional in L. pictus embryos. In contrast, the LpMT1 promoter is active in HeLa cells and in embryos of the sea urchin Strongylocentrotus purpuratus. The hMT-IIA gene may lack a cis-acting sequence element required for expression of MT genes in L. pictus embryos. The LpMT1 promoter is a powerful, inducible, promiscuous promoter useful for driving the expression of heterologous genes in sea urchin embryos.     

Functional analyses of promoter elements responsible for the differential expression of the human metallothionein (MT)-IG and MT-IF genes

The sequences responsible for heavy metal-inducible expression are situated within the proximal 437 and 160 base pairs (bp) of MT-IF and MT-IG 5'-flanking sequence, respectively. Only 105 bp of proximal MT-IG 5'-flanking sequence containing a TATA box, two metal responsive elements (MREs), and three GC motifs and 147 bp of proximal MT-IF 5'-flanking sequence containing a TATCA box, four MREs, and two GC motifs were required for heavy metal-inducible expression. However, the proximal 111 bp of MT-IF 5'-flanking sequences (a TATCA box, two MREs, and two GC motifs) was not responsive to heavy metals and competes less efficiently than the 105-bp MT-IG fragment in a competition transfection analysis. The MT-IF promoter fragment containing MREc and MREd is substantially stronger and a more efficient competitor than the MT-IG promoter fragment containing MREc and MREd. Furthermore, the proximal 160 bp of MT-IG 5'-flanking sequence functions as a strong metal-inducible promoter but not as a metal-inducible enhancer. Mobility shift analysis of MT-IF and MT-IG promoter subregions suggests a correlation between protein binding to MRE sequences and MT gene expression. These data illustrate that the overall structural and functional organization of the MT-IF and MT-IG promoters are very different and that the molecular mechanisms governing differential expression levels of human MT genes are quite complex.