The Tax protein of human T-lymphotropic virus (HTLV)-1 activates expression of the HTLV-1 long terminal repeat through a DNA element that resembles the cellular cyclic AMP-regulated enhancer (CRE). Tax contains a transcriptional activation domain, but its ability to activate gene expression depends on interactions with cellular CRE-binding proteins such as CREB. Whether Tax can activate the expression of cellular CRE-containing genes has been controversial. Here we show that Tax can activate both the HTLV-1 and consensus cellular CREs, and propose that this activation may occur through mechanisms that are differentially dependent on CREB phosphorylation. Tax not only increases the binding of CREB to the viral CRE but also recruits the transcriptional co-activator CBP in a manner independent of CREB phosphorylation. In contrast, association of Tax with the cellular CRE occurs through CBP which, in turn, is recruited only in the presence of phosphorylated CREB.

Adipocyte determination-and differentiation-dependent factor 1 (ADD1), a member of the basic helix-loophelix (bHLH) family of transcription factors, has been associated with both adipocyte differentiation and cholesterol homeostasis (in which case it has been termed SREBP1). Using PCR-amplified binding analysis, we demonstrate that ADD1/SREBP1 has dual DNA sequence specificity, binding to both an E-box motif (ATCACGTGA) and a non-E-box sequence previously shown to be important in cholesterol metabolism, sterol regulatory element 1 (SRE-1; ATCACCCCAC). The ADD1/SREBP1 consensus E-box site is similar to a regulatory sequence designated the carbohydrate response element, defined by its ability to regulate transcription in response to carbohydrate in genes involved in fatty acid and triglyceride metabolism in liver and fat. When expressed in fibroblasts, ADD1/SREBP1 activates transcription through both the carbohydrate response E-box element and SRE-1. Substitution of an atypical tyrosine in the basic region of ADD1/SREBP1 to an arginine found in most bHLH protein causes a restriction to only E-box binding. Conversely, substitution of a tyrosine for the equivalent arginine in another bHLH protein, upstream stimulatory factor, allows this factor to acquire a dual binding specificity similar to that of ADD1/SREBP1. Promoter activation by ADD1/SREBP1 through the carbohydrate response element E box is not sensitive to the tyrosine-to-arginine mutation, while activation through SRE-1 is completely suppressed. These data illustrate that ADD1/SREBP1 has dual DNA sequence specificity controlled by a single amino acid residue; this dual specificity may provide a novel mechanism to coordinate different pathways of lipid metabolism.The basic helix-loop-helix (bHLH) transcription factors regulate gene expression by binding to specific DNA sequences. The basic domain of these proteins controls DNA binding to sites with the consensus sequence CANNTG. This consensus sequence is referred to as the E-box motif and is present in the regulatory regions of many tissue-specific genes (6,8,13,27,29). The function of the basic domain in DNA binding has been illustrated by mutations that disrupt the interaction with DNA but not oligomerization (13, 34). The Id protein, which has no basic domain, does not bind DNA but can act as a dominant suppressor of DNA binding of certain other bHLH proteins (3, 16). The HLH domain mediates homo-and hetero-oligomerization through two amphipathic ␣ helices connected by a variable loop region (1, 3, 13, 29). The various bHLH proteins can be divided into at least three groups (30). These include the broadly expressed class A proteins (E12, E47, E2-2, and daughterless) (11, 22), the tissue-specific class B proteins (MyoD, myogenin, MRF4, and achaete-scute) (9, 14, 36), and class C proteins, which feature a tandem arrangement of bHLH and leucine zipper (LZ) motifs (c-Myc, Max, upstream stimulatory factor [USF], AP4, TFE3, and TFEB) (2,5,7,10,21,24).Adipocyte determination-and differentiation-dependent factor...

Diets high in simple carbohydrates and low in fats lead in the mammalian liver to induction of a set of enzymes involved in lipogenesis. This induction occurs, in part, through transcriptional mechanisms that lead to elevated levels of the mRNA for these enzymes. For most of the lipogenic enzymes, an increase in glucose metabolism is required to trigger the transcriptional response. The intracellular mediator of this signaling pathway is unknown, although evidence suggests either glucose-6-phosphate or xylulose-5-phosphate. Studies to map the regulatory sequences of lipogenic enzyme genes involved in the transcriptional response have been performed for the L-type pyruvate kinase, S14, and acetyl-coenzyme A carboxylase genes. These studies have identified the DNA sequences necessary to link the signal generated by carbohydrate metabolism to specific nuclear transcription factors.

CHC binds specifically to aurora A–phosphorylated TACC3, recruiting it to the spindle during mitosis.