labial

Promoter Structure

labial contains a TATA-box deficient (TATA-less) promoter. Such promoters have a conserved sequence motif, A/GGA/TCGTG, termed the downstream promoter element (DPE), which is located about 30 nucleotides downstream of the RNA start site of many TATA-less promoters, including labial. DNase I footprinting of the binding of epitope-tagged TFIID to TATA-less promoters reveals that the factor protects a region that extends from the initiation site sequence (about +1) to about 35 nucleotides downstream of the RNA start site. There is no such downstream DNase I protection induced by TFIID in promoters with TATA motifs. This suggests that the DPE acts in conjunction with the initiation site sequence to provide a binding site for TFIID in the absence of a TATA box to mediate transcription of TATA-less promoters (Burke, 1996).

labial is induced to high levels of localised expression in the endoderm of Drosophila embryos by the action of DPP. Dissection of LAB 5' flanking sequences reveals two types of response elements. One of these mediateslab dependent activity, providing evidence that lab induction in the endoderm is autoregulatory. The other element, to a large extent independent of lab function, responds to DPP (Tremml, 1992).

Examination of the Ultrabithorax midgut enhancer, for which the Dpp response element has been localized to the 95 bp DI-DII interval, and the labial endoderm enhancer reveals a Mothers against Dpp binding-site consensus of GCCGnCGC. The two sites of highest affinity match the MAD consensus perfectly, and three lower-affinity sites contain mismatches in one or as many as three positions (Kim, 1997).

The homeotic proteins encoded by the genes of the Drosophila HOM and the vertebrate HOX complexes do not bind divergent DNA sequences with a high selectivity. In vitro, HOM (HOX) specificity can be increased by the formation of heterodimers with Extradenticle (Exd) or PBX homeodomain proteins. A single essential Labial (Lab)/Exd-binding site has been identified in a Decapentaplegic (Dpp)-responsive enhancer of the homeotic gene lab, which drives expression in the developing midgut. Lab and Exd bind cooperatively to the site in vitro, and the expression of the enhancer in vivo requires exd and lab function. In addition, point mutations in either the Exd or the Lab subsite compromise enhancer function, strongly suggesting that Exd and Lab bind to this site in vivo. Interestingly, the activity of the enhancer is only significantly stimulated by Dpp signaling upon the binding of Lab and Exd. Thus, the enhancer appears to integrate positional information via the homeotic gene lab, and spatiotemporal information via Dpp signaling; only when these inputs act in concert in an endodermal cell is the enhancer fully active. These results illustrate how a tissue-specific response to Dpp can be generated through synergistic effects on an enhancer carrying both Dpp- and HOX-responsive sequences (Grieder, 1997).

How does Dpp signaling affect the expression of the lab550 enhancer? The enhancer has been reported as a Dpp response element based on the fact that all of its activity in the midgut requires dpp function, but very little of it is due to lab function; in addition, it drives expression one or two cells more posterior than the endogenous lab gene, thus apparently displaying lab-independent expression. Although these conclusions somewhat contrast with those presented in the current paper, further analysis has suggested that several cyclic AMP response elements (CREs) in the enhancer mediate the Dpp responsiveness, directly or indirectly. More recently, in vitro binding sites for Drosophila Mad (a member of the SMAD protein family) and thought to act as co-activators of transcription in the Dpp signaling pathway, have also been identified in the lab enhancer. In addition, there are numerous binding sites for Schnurri, a putative transcription factor that is required for endodermal cells to respond to Dpp (Grieder, 1997 and references).

Despite the presence of all these putative target sequences of the Dpp signaling pathway, the current paper demonstrates that the expression of the lab enhancer strongly depends on the presence of a 48 bp region that harbors an Exd/Lab site; in exd and lab mutant embryos, expression of the element is strongly reduced. Since it recently has been shown that the translocation of Exd from the cytoplasm to the nucleus in the endoderm is controlled by Dpp and Wingless (Wg) , and since the endogenous lab gene is strongly induced by Dpp, it is likely that part of the Dpp regulation of the lab enhancer enters through the Exd/Lab site. This is consistent with the current findings and with the previous observation that in the absence of lab function, dpp-mediated induction is hardly working or working with reduced efficiency (Grieder, 1997 and references).

What is the role of the additional Dpp response elements in the lab enhancer? Clearly, the activity of the enhancer also depends on the presence of CRE sites; similar to observations concerning the Lab/Exd site, mutations in the four CRE sites result in weaker expression of the enhancer. This suggests that on the full-length enhancer, the additional (direct or indirect) Dpp input via the CRE and other sites is required in concert with the input through the Lab/Exd site, and that the absence of either input impairs expression of the enhancer. Two elements appear to act synergistically with respect to their response to Dpp; only on the addition of a weak Dpp response element to the Lab/Exd-containing element is a cis-acting element generated which is strongly Dpp inducible (and Lab/Exd dependent). It is proposed that the strong Dpp responsiveness of the enhancer is limited, through synergistic interactions, to those cells in which the Lab/Exd site is occupied. The function of this dual requirement might be to insure that the enhancer activates transcription exclusively in cells located in the central portion of the midgut endoderm where low levels of Lab in the tip of the two endoderm primordia coincide with Dpp, secreted from the visceral mesoderm parasegment 7 (Grieder, 1997).

To regulate their target genes, the Hox proteins of Drosophila often bind to DNA as heterodimers with the homeodomain protein Extradenticle (Exd). For Exd to bind DNA, it must be in the nucleus, and its nuclear localization requires a third homeodomain protein, Homothorax (Hth). A conserved N-terminal domain of Hth directly binds to Exd in vitro, and is sufficient to induce the nuclear localization of Exd in vivo. However, mutating a key DNA binding residue in the Hth homeodomain abolishes many of its in vivo functions. Hth binds to DNA as part of a Hth/Hox/Exd trimeric complex, and this complex is essential for the activation of a natural Hox target enhancer. Using a dominant negative form of Hth evidence is provided that similar complexes are important for several Hox- and exd-mediated functions in vivo. These data suggest that Hox proteins often function as part of a multiprotein complex, composed of Hth, Hox, and Exd proteins, bound to DNA (Ryoo, 1999).

The tight interaction between Hth and Exd proteins, together with the requirement for the Hth homeodomain for many of Hthís functions, suggested that Hth might be binding to the same target enhancers as Hox/Exd heterodimers. One well characterized Hox/Exd target is an autoregulatory enhancer from the labial (lab) gene, called lab550. A 48 bp fragment of lab550, lab48/95, is necessary for lab550 activity and, in one copy, is sufficient to direct a labial- and exd-dependent pattern of expression in endodermal cells. In lab48/95 there is a single Lab/Exd heterodimer binding site, TGATGGATTG; this binding site is necessary for the activity of lab550. Also in lab48/95 is a binding site that resembles a high affinity site for MEIS1: GACTGTCA, a murine Hth homolog. To test if this site is a bona fide Hth binding site, band shift experiments were performed with Lab, Hth, and Exd proteins on the wild-type lab48/95 oligo, and on an oligo with point mutations in the putative Hth binding site, GACTtatA (lab48/95 hth). Neither Lab, Exd, nor Hth are able to bind lab48/95 on their own. The combination of Exd plus Hth is able to weakly bind this DNA. Because binding is diminished on lab48/95 hth, these data suggest that Exd and Hth exhibit weak cooperative binding to lab48/95, consistent with previous studies with MEIS1 and PBX1. Lab cooperatively binds with Exd to lab48/95 and the binding of this heterodimer requires both the Exd and Lab half sites. In contrast, no complex formation is observed when Hth and Lab are combined. However, when increasing amounts of Hth are added to a constant amount of Lab plus Exd, the Lab/Exd band disappears and in its place a Hth/Lab/Exd trimeric complex is observed. The Hth/Lab/Exd band is more intense than the Lab/Exd band, suggesting that Hth contributes to the DNA binding affinity of the trimeric complex. Additonal tests show that the Hth/Lab/Exd complex requires the putative Hth binding site; use of truncated proteins show that protein-protein interaction between Hth and Exd is necessary for the formation of the Hth/LAB/Exd complex, but that DNA binding by the Hth homeodomain contributes to the stability of this complex. Also, the Hth binding site is required for lab48/95 activity in embryos. Thus a DNA bound Hth/LAB/Exd triple complex is capable of activating lab48/95-lacZ in vivo. This was confirmed by interfering with the stable assembly of this complex by expressing the HM domain, which binds to Exd and therefore competes with the interaction between Exd and Hth (Ryoo, 1999).

During embryonic development of the Drosophila brain, the Hox gene labial is required for the regionalized specification of the tritocerebral neuromere. In order to gain further insight into the mechanisms of Hox gene action in the CNS, the molecular and genetic basis of cross-regulatory interactions between labial and other more posterior Hox genes were examined using the GAL4/UAS system for targeted misexpression. Misexpression of posterior Hox genes in the embryonic neuroectoderm results in a labial loss-of function phenotype and a corresponding lack of Labial protein expression in the tritocerebrum. This is due to repression of labial gene transcription in the embryonic brain. Enhancer analysis suggests that this transcriptional repression operates on a 3.65 kb brain-specific labial-enhancer element. A functional analysis of Antennapedia and Ultrabithorax protein domains shows that the transcriptional repression of labial requires homeodomain-DNA interactions but is not dependent on a functional hexapeptide (a conserved stretch of aminoacids that is found in many Hox proteins and is involved in interactions between Hox proteins and Exd). The repressive activity of a Hox protein on labial expression in the tritocerebrum can, however, be abolished by concomitant misexpression of a Hox protein and the cofactors Homothorax and nuclear-targeted Extradenticle. Taken together, these results provide novel and detailed insight into the cross-regulatory interactions of Hox genes in embryonic brain development and suggest that specification of tritocerebral neuronal identity requires equilibrated levels of a Hox protein and Hth and nuclear-targeted Exd (n-Exd) cofactors (Sprecher, 2004).

The main function of Hox genes is to assign positional identities along the embryonic body axis in animals ranging from arthropods to vertebrates. Several mechanistic paradigms have been proposed to describe Hox gene action, two of which are the concepts of cross-regulation among Hox genes, and of co-operative interactions between Hox genes and protein cofactors. In the developing CNS of Drosophila, loss-of-function studies have shown that Hox genes expressed in more posterior regions act as negative regulators of Hox genes that are expressed in more anterior regions of the CNS. For example Antp is primarily expressed in Parasegment (PS) 4 and PS5 of the CNS, but it is also expressed at lower levels in PS6-13. In embryos that lack the Bithorax-complex genes, Antp expression is high in PS4-13, suggesting that BX-C gene action keeps Antp expression low in PS6-13. Similarly, BX-C genes that are expressed and function in more posterior abdominal segments keep Ubx expression low in PS7-13. In the absence of the abdominal BX-C genes, Ubx products are found at high levels in PS6-13. In addition, recent gain-of-function experiments have shown that ectopic Ubx and Abd-A are able to repress lab and Scr in the CNS in a timing dependent manner while otherwise overlapping expression of other Hox genes is tolerated (Sprecher, 2004).

In this analysis, focus was placed on lab, the Hox gene specifically expressed in the tritocerebral neuromere. Genetic analyses have shown that lab is essential for the acquisition of neuronal identity in its tritocerebral expression domain, and lab loss-of-function mutations leads to severe defects in the establishment of the tritocerebral neuromere. The action of lab in this domain can be eliminated by targeted misexpression of posterior Hox genes through the sca::Gal4 driver, resulting in a lab loss-of-function phenotype in the brain. This suppression of lab action has a number of features that are characteristic of the type of cross-regulatory Hox gene interactions that have been demonstrated in developing epidermal structures. (1) The suppression of lab in the tritocerebrum appears to be time dependent. While early misexpression of posterior Hox genes during neuroectoderm specification and neuroblast formation at embryonic stage 9 reliably results in lab suppression in the tritocerebrum, later misexpression, after embryonic stage 10/11, does not. (2) lab suppression by misexpression of posterior Hox genes is tissue specific. Thus, while Hox gene misexpression via the sca::Gal4 driver suppresses lab expression in the tritocerebrum, it augments lab expression in the endodermal cells of the midgut. (3) Misexpression of posterior Hox genes leads to a loss of Lab protein in the affected domain, and this lack of Lab is in accordance with the observed phenocopy of a lab loss-of-function mutation observed in this domain (Sprecher, 2004).

In several respects these experiments extend insights into cross-regulatory interactions beyond the observations made on developing epidermal structures. Evidence suggests that the suppression of lab by a posterior Hox gene like Ubx is due to transcriptional repression. Thus, in sca::Gal4/UAS::Ubx embryos, lab transcripts disappear and are absent in the developing tritocerebrum from stage 10/11 onward. This tritocerebrum-specific repression appears to be mediated through a 3.65 kb enhancer element upstream of the lab gene transcriptional start site. Moreover, the results imply that suppression of lab in the developing tritocerebrum by posterior Hox genes requires a functional homeodomain; mutations of the homeodomain in the Antp gene abolish the repressive activity of this Hox gene. In addition, the findings indicate that the suppressive cross-regulatory action of a posterior Hox gene like Ubx is not dependent on a functional hexapeptide. Thus, misexpression of a UAS::UbxYAAA transgene in which the critical YPWM motif of Ubx was mutated to the sequence YAAA, still results in complete suppression of lab in the developing tritocerebrum. Finally, evidence is provided that concomitant misexpression of Ubx, nuclear-targeted Exd and Hth is able to completely rescue the lab loss-of-function mutant phenotype. This implies that the Exd and Hth cofactors can switch Ubx protein action between different functional states in which Exd and Hth are required for Hox protein transcriptional activation functions whereas they are dispensable for Hox transcriptional repression functions. Moreover, these findings can be explained by models in which the hexapeptide is involved in the regulation of Hox protein activity, and may also reflect a requirement for equilibrated levels of a Hox gene product and the Hth and n-Exd cofactors in the specification of tritocerebral identity (Sprecher, 2004).

Transcriptional regulation, targets of activity, protein interactions and phenotypic suppression

Continued: Regulation part 2/3 | part 3/3

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