Gene name - hunchback
Cytological map position - 85A3-B1
Function - transcription factor
Keywords - morphogen - anterior-posterior axis and gap gene
Symbol - hb
Genetic map position - 3-48.3
Classification - zinc finger protein (C2H2)
Cellular location - nuclear
The function of hunchback is central to the establishment of an anterior-posterior gradient of gene activity in the transition from unfertilized egg to developing zygote. As its name suggests, hunchback has a special role in the development of the trunk (thorax) of the fly.
Maternal HB mRNA, is intitially distributed evenly throughout the egg. Nanos, whose mRNA is localized to the posterior pole of mature oocyte, functions to inhibit Hunchback: the Nanos protein inactivates HB mRNA, preventing its translation in the posterior. Thus Nanos, through its inhibition of HB translation, establishes a concentration gradient of maternally derived HB protein complementary to the gradient of Nanos protein (Pelegri, 1994). It is not Nanos itself that binds to the Nanos response elements of HB mRNA, but rather another protein, Pumilio, that apparently recruits Nanos into a multiprotein-RNA complex (Murata, 1995).
After fertilization, maternally derived Hunchback is supplanted by a zygotic HB transcript. Transcription is driven by Bicoid in the anterior. Bicoid is arrayed in an anterior to posterior gradient, and activates hunchback expression along this gradient, giving rise to an anterior-posterior Hunchback zygotic gradient.
Hunchback acts both to activate anterior gap gene function as a co-activator with Bicoid, and to shift the effective morphogenetic activity of Bicoid toward the posterior, thus extending the effective range of Bicoid (Simpson-Brose, 1994). Hunchback can operate both as a transcription activator or repressor, and as such determines the placement of both anterior and posterior gap genes. Hunchback's main role is as a repressor of posterior gap gene expression in the anterior. Krüppel expression in the middle of the embryo is regulated by HB. knirps and giant are expressed in the posterior, but these genes are repressed in the anterior by Hunchback.
Enhancer of zeste( E[z]) is required to maintain transcriptional repression of knirps and giant once repression has been initiated by Hunchback. A role for Polycomb group genes in the regulation of gap genes is a fairly recent idea; it is now apparent that Hunchback and E(z) act together at the same cis-acting sequences to establish repression in the knirps promoter (Pelegri, 1994).
Hunchback activity in the posterior is regulated by Tailless and Huckebein (Margolis, 1995). hunchback acts like a gap gene in the posterior. Mutants evince fused 7th and 8th segments [Images] (Tautz, 1987). Perhaps Hunchback acts as a cofactor with Krüppel and Knirps. It has been demonstrated that HB can associate with these gap gene products and that their interaction results in gene repression (Sauer, 1995).
While the Bcd gradient has served as a model system in understanding pattern formation in Drosophila, it is suspected that this is not the case in more ancestral insects. The long-germ mode of development as found in Drosophila is probably an adaptation to its particularly rapid embryogenesis. The ancestral type of embryogenesis in insects and arthropods is the short germ type. In these embryos, the germ rudiment forms at the posterior ventral side of the egg. In extreme cases like the grasshopper, it may be restricted to only a few percent of the total egg length - which makes it difficult to imagine how an anteriorly localized BCD mRNA could determine pattern formation at the posterior end of the egg. Moreover, classical experiments have only yielded evidence for a posteriorly localized organizing activity. Therefore, bcd could be considered a late addition during insect evolution and its pivotal function during embryogenesis could be restricted to higher dipterans. This paper is concerned with early pattern formation of the flour beetle Tribolium castaneum. Tribolium is a typical example for short germ embryogenesis, representing the ancestral type of embryogenesis in insects, albeit not in its extreme form, like the grasshopper (see Tribolium early embryonic development). In contrast to Drosophila, only cephalic and thoracic segments, but not abdominal segments, are determined during the blastoderm stage. Furthermore, the most anterior 20% of the Tribolium blastoderm cells form an extra-embryonic membrane, the serosa. This structure is not found in this form in higher Dipterans like Drosophila, but is again an ancestral feature of insect embryogenesis. Prior to gastrulation, most blastoderm cells move from anterior and dorsal positions towards the posterior ventral region where they form the embryo proper. This germ rudiment then continues to grow from its posterior end to form a germ band which eventually encompasses all abdominal segments (Wolff, 1998).
Thus, in short germ embryos, the germ rudiment forms at the posterior ventral side of the egg, while the anterior-dorsal region becomes the extra-embryonic serosa. It is difficult to see how in these embryos an anterior gradient like that of Bicoid protein in Drosophila could be directly involved in patterning of the germ rudiment. Moreover, since it has not yet been possible to recover a bicoid homolog from any species outside the diptera, it has been speculated that the anterior Bicoid gradient could be a late addition during insect evolution. This question was addressed by analyzing the regulation of potential target genes of bicoid in the short germ embryo of Tribolium castaneum. Homologs of caudal and hunchback from Tribolium are regulated by Drosophila bicoid. In Drosophila, maternal Caudal mRNA is translationally repressed by Bicoid. Tribolium Caudal RNA is also translationally repressed by Bicoid, when it is transferred into Drosophila embryos under a maternal promoter. This strongly suggests that a functional bicoid homolog must exist in Tribolium. The second target gene, hunchback, is transcriptionally activated by Bicoid in Drosophila. Transfer of the regulatory region of Tribolium hunchback into Drosophila also results in regulation by early maternal factors, including Bicoid, but in a pattern that is more reminiscent of Tribolium hunchback expression, namely in two early blastoderm domains. Using enhancer mapping constructs and footprinting, it has been shown that Caudal activates the posterior of these domains via a specific promoter. These experiments suggest that a major event in the evolutionary transition from short to long germ embryogenesis was the switch from activation of the hunchback gap domain by Caudal to direct activation by Bicoid. This regulatory switch can explain how this domain shifted from a posterior location in short germ embryos to its anterior position in long germ insects, and it also suggests how an anterior gradient can pattern the germ rudiment in short germ embryos, i.e. by regulating the expression of caudal (Wolff, 1998).
The key to understanding the qualitative switch that took place in insect evolution is believed to lie in the more anterior serosa expression domain of Tribolium hb. Reporter gene data suggest that this domain may already be activated by Bcd in Tribolium. To explain the switch in the regulation of the more posterior gap domain of hb expression, one can envision an intermediate state, where the serosa domain and the embryonic (gap) domain have fused into a single domain. To achieve this, the evolution of a few additional Bcd binding sites in the hb upstream region would have been sufficient. In this intermediate stage both Bcd and Cad could have acted as activators on the gap domain of hb. Subsequent loss of Cad regulation would then have moved the posterior boundary of this combined domain towards the anterior. It is noted that the Tribolium hb gene has three known promoters, one of which appears to be specialized for mediating Cad regulation. In Drosophila, only two promoters are present, neither of which has a known responsiveness to Cad. Thus, in all likelihood, the Cad dependent promoter and its associated enhancer was lost. Since no other enhancer activity has been found for later expression patterns of hb in the cad dependent fragment, the loss of this region could have been a single step. Intriguingly, a combined serosa and gap domain is still evident in the lower dipteran Clogmia. In this fly, hb is expressed in a large anterior domain, from which at later stages also the serosa is recruited (Rohr, personal communication to Wolff, 1998). This mechanism, the modification of the way gap genes sense maternal positional information while this information itself remains constant, can explain how the blastoderm fate map changed during evolution of short germ insects to insects with long germ embryos. Moreover, it represents an intriguing example for the importance of regulatory adaptation during the evolution of developmental processes (Wolff, 1998).
In addition to its early regulatory functions during segmentation, Hunchback is also expressed in the developing nervous system (see Lateral views of Drosophila CNS). One possible CNS regulatory target for Hb is the POU gene pdm-1. Hb regulates pdm-1 expression at the cellular blastoderm stage (Lloyd, 1991; Cockerill, 1993), and may play a similar role in the CNS. Since Hb and Castor bind similar promoter target sequences, an exploration was carried out of the embryonic distribution of the three proteins using polyclonal antibodies. It is suggested that Hb and Cas act in a cooperative, non-overlapping manner to control POU gene expression during Drosophila CNS development. By silencing pdm expression in early and late NB sublineages, Hb and Cas establish three pan-CNS compartments whose cellular constituents are marked by the expression of either Hb, Pdm, or Cas. During the initial S1 and S2 waves of NB delaminations, Pdm-1 is expressed in most, if not all, neuroectoderm cells. However, no Pdm-1 is detected in fully delaminated NBs and during stage 9 only a small subset of ventral cord GMCs express detectable levels. At this time, Hb expression is detected in all fully delaminated NBs and in many of their GMCs but not in neuroectoderm cells. Starting at late stage 9, Hb immunoreactivity is progressively lost from NBs; by late stage 10 only a small subset of ventral cord NBs express Hb. However, Hb is detected in many GMC and in their progeny generated during the first rounds of GMC production. These early sublineages reside predominantly along the inner/dorsal surfaces of the developing ganglia. The reduction in Hb NB expression coincides with the activation of Pdm-1 NB expression; by late stage 10, Pdm-1 is detected in many cephalic lobe (see Views of cephalic lobe neuroblasts) and ventral cord NBs and in GMCs. Similar to the dynamics of Hb expression, Pdm-1 NB expression is transient. However, many GMCs and their progeny arising from the Pdm-expressing NBs maintain high levels of Pdm-1. (Kambadur, 1998).
Bases in 5' UTR - 221 for P1 and 152 for P2
Exons - two
Hunchback and Krüppel are homologous; they share four zinc finger domains. Hunchback has a higher molecular weight than Krüppel, because of an additional two zinc fingers at its C-terminal end (Tautz, 1987). A subset of zinc finger transcription factors contain amino acid sequences that resemble those of Krüppel. They are characterized by multiple zinc fingers containing the conserved sequence CX2CX3FX5LX2HX3H (X is any amino acid, and the cysteine and histidine residues are involved in the coordination of zinc) that are separated from each other by a highly conserved 7-amino acid inter-finger spacer, TGEKP(Y/F)X, often referred to as the H/C link.
Each 30-residue zinc finger motif folds to form an independent domain with a single zinc ion tetrahedrally coordinated beween an irregular, antiparallel, two stranded ß-sheet and a short alpha-helix. Each zinc finger of mouse Zif268 (which has three fingers) binds to DNA with the amino terminus of its helix angled down into the major groove. An important contact between the first of the two histidine zinc ligands and the phosphate backbone of the DNA contributes to fixing the orientation of the recognition helix. Although the two fingers of Drosophila Tramtrack interact with DNA in a way very similar to those of Zif268, there are important differences. Tramtrack has an additional amino-terminal ß-strand in the first of the three zinc fingers. The charge-relay zinc-histidine-phosphate contact of Zif268 is substituted by a tyrosine-phosphate contact. In addition, for TTK, the DNA is somewhat distorted with two 20 degree bends. This distortion is correlated with changes from the rather simple periodic pattern of amino base contacts seen in Zif268 and finger 2 of TTK (Klug, 1995 and references).
Castor, a transcription factor with similar DNA binding specificity to that of Hb, contains a centrally located Zn-finger domain made-up of four consecutive C2-H2C2-H2 repeats. The second C2-H2 of each repeat closely resembles fingers of the Xenopus TFIIIA C2-H2 class. Flanking this repeat are motifs that may constitute either transcription transactivation or repression domains. UV induced protein-DNA cross-linking in vivo studies reveal that Cas binds genomic DNA. To determine if Cas is a sequence-specific DNA-binding protein, the cyclic amplification of selected targets protocol was used. After six rounds of selection/amplification, sequencing of cloned fragments revealed that all had at least one sequence motif in common and some contained two core recognition sequences. DNA fragments containing one site homologous to the consensus site produce a single prominent Cas-DNA gel-shift; a fragment with two, generates two complexes. Addition of Cas-specific antisera causes a super-shift of the Cas-DNA complex. A search of known transcription factor DNA-binding sites shows that the Cas recognition sequence is almost identical to that of the Drosophila Zn-finger protein Hunchback. The Cas consensus matches 9 out of 10 bp for the reported Hb sites. To determine if Cas binds Hb sites, gel-shift experiments using DNA fragments were carried out with exact sequence matches to Hb targets. Cas does indeed bind to these sites. The sequence-specificity of Cas-DNA binding to Hb recognition sites was further tested by competition assays and base-pair substitutions. Taken together, these experiments demonstrate that Cas can bind to the same DNA sites as Hb, raising the possibility that it modulates transcriptional activities of genes also regulated by Hb. Secondary structure predictions of the Cas finger domain indicate that only the first and third of its TFIIIA-like fingers contain alpha-helices. Interestingly, optimal alignment of Cas and Hb fingers reveals that the first and third a-helices of Cas share the highest homology with the corresponding a-helices of Hb (33% identity for the first and 27% for the third). Although speculative, their shared DNA-binding preferences may be due in part to the shared residues found in these predicted reading heads. Outside of their Zn-fingers, Hb and Cas show no obvious sequence similarities (Kambadur, 1998).
date revised: 1 Oct 98
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