Gene name - tango
Synonyms - Arnt
Cytological map position - 85C
Function - Transcription factor
Symbol - tgo
FlyBase ID: FBgn0264075
Genetic map position -
Classification - Myc-type, helix loop helix and PAS family protein.
Cellular location - nuclear and cytoplasmic
The cloning of tango also known as Drosophila Arnt, a homolog of the vertebrate Ah receptor nuclear translocator (Arnt) was reported simultaneously by three different laboratories (Zelzer, 1997, Ohshiro, 1997 and Sonnenfeld, 1997). Tango heterodimerizes with two transcription factors, Trachealess (Trh) and Single minded (Sim), to regulate transcription in the trachea and central midline, respectively. Tango, like Trh and Sim, is a bHLH-PAS transcription factor; all are able to form heterodimers via interaction through their PAS domains (Zelzer, 1997).
Using several criteria, biochemical studies provide evidence that Tango associates with Sim and Trh. (1)Tgo protein is expressed in all cells in the embryo and thus overlaps in expression with Sim in the CNS midline cells and Trh in tracheal cells. (2) Studies using the yeast two-hybrid assay indicate that Tango can form dimers with Sim, Trh and Sima (a Drosophila gene related to mammalian hypoxia-indicible factor 1alpha [Nambu, 1996]). (3) Co-immunoprecipitation experiments with caculoviral-expressing proteins indicate that Tango forms dimers with Sim and Trh. (4) Expression studies in cultured cells suggest two pairwise interactions: Sim-Tgo and Trh-Tgo, with either of the two pair capable of interaction in order to bind DNA and activate transcription. (5) Mutations in tango result in CNS midline and tracheal defects, defects similar to those observed with mutations in sim and trh, respectively. (6) Gene dosage experiments suggest that the two pairs (sim-tgo and trh-tgo) interact to control CNS midline and tracheal cell transformation and development, respectively (Sonnenfeld, 1997).
The development of Drosophila trachea is under the control of spatially and/or quantitatively regulated activity involving the FGF receptor known as Breathless, which is also essential for midline glial migration. Examination of the proximal promoter region of the breathless promoter reveals three conserved elements (central midline elements or CMEs) resembling previously identified putative binding sites for Sim/Arnt heterodimers (Swanson, 1995) within a 150 base pair region, from -606 to -447 bases, relative to the P2 transcriptional initiation site. These three sites account for breathless expression in midline precursor cells (Ohshiro, 1997).
breathless expression in developing trachea is regulated by direct interactions between Trachealess/Tango heterodimers and three identical central midline elements (TACGTGs) situated in the minimum enhancer region. To test whether the heterodimer of Tango and Trh is capable of binding to CMEs, Trh and Tango fusion proteins were subjected to electrophoretic mobility shift assays of a CME1-containing oligonucleotide. In the absence of either Tango or Trh or both, there is little or nor protein-DNA interaction. In contrast, a retardation band can be seen when the oligonucleotide is incubated with a reaction mixture containing both Tango and Trh, indicating that Trh and Tango are capable of forming a heterodimer which exhibits DNA-binding activity (Ohshiro, 1997).
Beside regulation of central midline and tracheal development, there are likely to be additional roles for Tango. Tango interacts biochemically with the Drosophila Sima protein. The function of Sima is currently unknown, but its ubiuquitous expression pattern and primary sequence suggests it may be related functionally to mammalian HIF-1alpha; it may control the Drosophila response to hypoxia. Drosophila cell culture experiments have revealed the existence of a CME-binding factor that is induced under hypoxic conditions (Nagao, 1996). Therefore, Tango could be involved in controlling the hypoxia response.
The Drosophila spineless (ss) gene encodes a basic-helix-loop-helix-PAS transcription factor that is required for proper specification of distal antennal identity, establishment of the tarsal regions of the legs, and normal bristle growth. ss is the closest known homolog of the mammalian aryl hydrocarbon receptor (Ahr), also known as the dioxin receptor. Dioxin and other aryl hydrocarbons bind to the PAS domain of Ahr, causing Ahr to translocate to the nucleus, where it dimerizes with another bHLH-PAS protein, the aryl hydrocarbon receptor nuclear translocator (Arnt). Ahr:Arnt heterodimers then activate transcription of target genes that encode enzymes involved in metabolizing aryl hydrocarbons. Ss functions as a heterodimer with the Drosophila ortholog of Arnt, Tango. The ss and tgo genes have a close functional relationship: loss-of-function alleles of tgo were recovered as dominant enhancers of a ss mutation, and tgo-mutant somatic clones show antennal, leg, and bristle defects almost identical to those caused by ss minus mutations. The results of yeast two-hybrid assays indicate that the Ss and Tgo proteins interact directly, presumably by forming heterodimers. Coexpression of Ss and Tgo in Drosophila SL2 cells causes transcriptional activation of reporters containing mammalian Ahr:Arnt response elements, indicating that Ss:Tgo heterodimers are very similar to Ahr:Arnt heterodimers in DNA-binding specificity and transcriptional activation ability. During embryogenesis, Tgo is localized to the nucleus at sites of ss expression. This localization is lost in a ss null mutant, suggesting that Tgo requires heterodimerization for translocation to the nucleus (Emmons, 1999).
Ectopic expression of ss causes coincident ectopic nuclear localization of Tgo, independent of cell type or developmental stage. In the embryo, ss is expressed in the antennal segment, the gnathal segments, the leg anlage, and the peripheral nervous system. Strong nuclear accumulation of Tgo is seen in the antennal segment, which expresses the highest level of ss. Nuclear accumulation of Tgo is also observed in the gnathal segments (mandibular, maxillary, and labial), but the intensity of staining is relatively weak compared to the antennal segment. This correlates with the relatively weak expression of ss in the gnathal segments, when compared to the antennal segment. Nuclear localization of Tgo in the antennal and gnathal segments is dependent on ss, since it is not seen in a ss null mutant. The expression of ss in the appendage primordia and the peripheral nervous system also correlates with Tgo nuclear accumulation. Sensory cells that express ss are in close proximity to the tracheal cells that express trh. To distinguish these, embryos were labeled with anti-Trh and anti-Tgo. Non-tracheal cells that show nuclear Tgo are observed in the location of ss-expressing sensory cells. This non-tracheal Tgo nuclear accumulation is absent in ss mutant embryos. These results indicate that Tgo accumulates in the nuclei of ss-expressing antennal, gnathal and sensory cells, consistent with the formation and nuclear accumulation of Ss:Tgo heterodimers in vivo. Surprisingly, no significant Tgo nuclear accumulation is seen in the limb primordia, even though ss is expressed in these cells. This may reflect regulatory events idiosyncratic to the limb primordia, or a lack of sensitivity of the immunostaining, since the limb primordia express ss at considerably lower levels than the antennal segment. Tgo nuclear accumulation is also observed in the cells of the dorsal vessel. Since, sim, ss and trh are not expressed in the dorsal vessel, an additional bHLH-PAS protein may function in combination with Tgo in controlling the development or physiology of these cells, which comprise the Drosophila circulatory system. When ectopic expression of a UAS-ss transgene is driven by en-Gal4, Tgo is found to accumulate in nuclei in circumferential ectodermal en stripes. Similarly, expression of ss in mesodermal cells (driven by twi-GAL4) causes nuclear accumulation of Tgo in the mesoderm. These experiments support the conclusion that Ss and Tgo interact in vivo, and suggest that their interaction and nuclear accumulation does not depend on additional, spatially-restricted, factors. Despite the very different biological roles of Ahr and Arnt in insects and mammals, the molecular mechanisms by which these proteins function appear to be largely conserved (Emmons, 1999).
How did Ss and Ahr come to have such different functions in vertebrates and arthropods? One possibility is that Ahr functioned as some type of chemosensory protein in an ancestral organism. In vertebrates, this function became utilized by all cells to sense aryl hydrocarbon toxins, whereas in arthropods it became intimately associated with the specification of a major chemosensory organ, the antenna. It is hoped that studies of organisms from other lineages will shed light on how Ss and Ahr came to adopt such different roles (Emmons, 1999).
tango maps to a site immediately adjacent to neuralized. Transcriptional orientation for the two genes is in the same direction: neuralized is oriented upstream of tango (Sonnenfeld, 1997).
The sequence reported by Zelzer (1997) differs in the C-terminal amino acids from that reported by Sonnenfeld (1997) and Ohshiro (1997). Tango is highly related to mammalian Arnt, both in sequence and predicted structure. The bHLH region is near the N-terminus, a feature common to all bHLH-PAS proteins, followed closely by the PAS domain, and then glutamine-rich C-terminal domains. The bHLH regions are 92% identical and PAS domains 53%. All three amino acids in the basic-region (His 18, Glu 22 and Arg 26)are invariant and presumed to be directly involved in DNA-protein contacts (Ohshiro, 1997). The C-terminal regions are generally unrelated in primary sequence, but share the occurrence of glutamine-rich sequences (18% in Tgo), which are activation domains in mammalian Arnt, Drosophila Sim, and may other transcription factors, and the entire regions is proline rich (15% in Tango. In particular, the polyglutamine regions are closely flanked by proline residues. One interesting feature of the Tango C-terminus not found in other Arnt proteins is the presence of a histidine-proline-rich region of unknown function found in a small number of other Drosophila transcription factors including the Paired segment polarity and Bicoid homeobox proteins. This regions is called the Paired (PRD) repeat, and is distinct from the 'Paired domain' (Sonnenfeld, 1997).
date revised: 8 February 98
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