Heike Taubert

Mechanism and Bicoid‐dependent control of hairy stripe 7 expression in the posterior region of the Drosophila embryo

Pair‐rule gene hairy (h) expression in seven evenly spaced stripes, along the longitudinal axis of the Drosophila blastoderm embryo, is mediated by a modular array of separate stripe enhancer elements. The minimal enhancer element, which generates reporter gene expression in place of the most posterior h stripe 7 (h7‐element), contains a dense array of binding sites for factors providing the trans‐acting control of h stripe 7 expression as revealed by genetic analyses. The h7‐element mediates position‐dependent gene expression by sensing region‐specific combinations and concentrations of both the maternal homeodomain transcriptional activators, Caudal and Bicoid, and of transcriptional repressors encoded by locally expressed zygotic gap genes. Caudal and Bicoid, which form complementing concentration gradients along the longitudinal axis of the embryo, function as redundant activators, indicating that the anterior determinant Bicoid is able to activate gene expression in the most posterior region of the embryo. The spatial limits of the h stripe 7 domain are brought about by the local activities of repressors which prevent activation. The results suggest that the gradients of Bicoid and Caudal combine their activities to activate segmentation genes along the entire axis of the embryo.

Formation of the drosophila larval photoreceptor organ and its neuronal differentiation require continuous Krüppel gene activity

The Drosophila segmentation gene Krüppel (Kr) is redeployed to play a critical role for the establishment of the larval visual system. Using reporter gene expression conducted by a specific Kr cis-acting element, we were able to trace back the origin of the larval photoreceptor organ, the Bolwig organ, to a single progenitor neuron and to examine Kr function in Bolwig organ development when Kr+ activity is absent from embryos due to specific mutations or reduced by neuron-specific and temporally restricted Kr antisense RNA expression. Our results show that Kr is required for neurons to differentiate into Bolwig organs, for fasciculation of the Bolwig nerve, and for this nerve to follow a specific pathway toward the synaptic targets in the larval brain. The transcription factor encoded by Kr is likely to regulate surface molecules necessary for neuronal cell adhesion and recognition in the developing larval visual system.

Embryonic expression and characterization of a PTx1 homolog in Drosophila

We describe the molecular characterization of the paired-type homeobox gene D-Ptx1 of Drosophila, a close homolog of the mouse pituitary homeobox gene Ptx1 and the unc-30 gene of C. elegans, characterized by a lysine residue at position 9 of the third alpha-helix of the homeodomain. D-Ptx1 is expressed at various restricted locations throughout embryogenesis. Initial expression of D-Ptx1 in the posterior-most region of the blastoderm embryo is controlled by fork head activity in response to the activated Ras/Raf signaling pathway. During later stages of embryonic development. D-Ptx1 transcripts and protein accumulate in the posterior portion of the midgut, in the developing Malpighian tubules, in a subset of ventral somatic muscles, and in neural cells. Phenotypic analysis of gain-of-function and lack-of-function mutant embryos show that the D-Ptx1 gene is not involved in morphologically apparent differentiation processes. We conclude that D-Ptx1 is more likely to control physiological cell functions than pattern formation during Drosophila embryogenesis.

Regulation of Drosophila spalt gene expression

The region-specific homeotic gene spalt is involved in the specification of terminal versus trunk structures during early Drosophila embryogenesis. Later in development spalt activity participates in specific processes during organogenesis and larval imaginal disc development. The multiple functions of spalt are reflected in distinct spatio-temporal expression patterns throughout development. Here we show that spalt cis-regulatory sequences for region-specific and organ-specific expression are clustered. Their organization may provide the structural basis for the diversification of expression pattern within the spalt/spalt related/spalt adjacent gene complex. We also examined the transacting factor requirement for the blastodermal spalt expression domains. They are under the genetic control of maternal and gap gene products and we show that these products are able to bind to corresponding spalt cis-acting sequences in vitro. The results suggest that the transacting factors, as defined by genetic studies, functionally interact with the spalt regulatory region. In addition, we provide evidence that a zygotic gene product of the terminal system, Tailless, cooperates with the maternal gene product Caudal and thereby activates gene expression in the terminal region of the embryo.

Phenotypic suppression of empty spiracles is prevented by buttonhead.

Unlike the trunk segments, the anterior head segments of Drosophila are formed in the absence of pair-rule and HOX-cluster gene expression, by the activities of the gap-like genes orthodenticle (otd), empty spiracles (ems) and buttonhead (btd). The products of these genes are transcription factors, but only EMS has a HOX-like homeodomain. Indeed, ems can confer identity to trunk segments when other HOX-cluster gene activities are absent. In trunk segments of wild-type embryos, however, ems activity is prevented by phenotypic suppression, in which more posterior HOX-cluster genes inactivate the more anterior without affecting transcription or translation. ems is suppressed by all other Hox-cluster genes and so is placed at the bottom of their hierarchy. Here we show that misexpression of EMS in the head transforms segment identity in a btd-dependent manner, that misexpression of BTD in the trunk causes ems-dependent structures to develop, and that EMS and BTD interact in vitro. The data indicate that this interaction may allow ems to escape from the bottom of the HOX-cluster gene hierarchy and cause a dominant switch of homeotic prevalence in the anterior–posterior direction.

Kruppel, a Drosophila segmentation gene, participates in the specification of neurons and glial cells

We report that the Drosophila segmentation gene Krüppel (Kr) is expressed in neural precursor cells, neurons and glial cells at different stages of neurogenesis and that Kr mutants develop aberrant peripheral (PNS) and central (CNS) nervous systems. Expression derived from a Kr minigene rescues the segmentation defects but these embryos continue to lack most of the neural Kr activity. Phenotypic analysis of the rescued embryos indicates that, in addition to overall effects on the PNS and CNS structure via its segmentation role, Kr expression in the nervous system is functionally required for establishing particular neural and glial fates.

Receptor tyrosine kinase signaling regulates different modes of Groucho-dependent control of Dorsal.

Transcriptional control of the Drosophila terminal gap gene huckebein (hkb) depends on Torso (Tor) receptor tyrosine kinase (RTK) signaling and the Rel/NFkappaB homolog Dorsal (DI). DI acts as an intrinsic transcriptional activator in the ventral region of the embryo, but under certain conditions, such as when it is associated with the non-DNA-binding co-repressor Groucho (Gro), it is converted into a repressor. Gro is recruited to the enhancer element in the vicinity of DI by sequence-specific transcription factors such as Dead Ringer (Dri). We examined the interplay between DI, Gro and Dri on the hkb enhancer and show that when acting over a distance, Gro abolishes rather than converts DI activator function. Reducing the distance between DI- and Dri-binding sites, however, switches DI into a Gro-dependent repressor that overrides activation of transcription. Both of the distance-dependent regulatory options of Gro - quenching and silencing of transcription - are inhibited by RTK signaling. These data describe a newly identified mode of function for Gro when acting in concert with DI. RTK signaling provides a way of modulating DI function by interfering either with Gro activity or with Dri-dependent recruitment of Gro to the enhancer.

Activation of posterior pair-rule stripe expression in response to maternal caudal and zygotic knirps activities.

Drosophila pair-rule gene expression, in an array of seven evenly spaced stripes along the anterior-posterior axis of the blastoderm embryo, is controlled by distinct cis-acting stripe elements. In the anterior region, such elements mediate transcriptional activation in response to the maternal concentration gradient of the anterior determinant BICOID and repression by spatially distinct activities of zygotic gap genes. In the posterior region, activation of hairy stripe 6 has been shown to depend on the activity of the gap gene knirps, suggesting that posterior stripe expression is exclusively controlled by zygotic regulators. Here we show that the zygotic activation of hairy stripe 6 expression is preceded by activation in response to maternal caudal activity. Thus, transcriptional activation of posterior stripe expression is likely to be controlled by maternal and zygotic factors as has been observed for anterior stripes. The results suggest that activation and the expression level mediated by the hairy stripe 6-element depend on the number of activator binding sites, likely to involve additive rather than synergistic interactions. We found an identical transacting factor requirement for hairy stripe 6 and 7 expression. The arrangement of the corresponding binding sites for the common factors involved in the control of the two stripes share a high degree of similarity, but some of the factors exert opposite regulatory functions within the two enhancer elements.

A two-step mode of stripe formation in the Drosophila blastoderm requires interactions among primary pair rule genes.

The stripe pattern of pair rule gene expression along the anterior-posterior axis of the Drosophila blastoderm embryo represents the first sign of periodicity during the process of segmentation. Striped gene expression can be mediated by distinct cis-acting elements that give rise to individual stripe expression domains in direct response to maternal and first zygotic factors. Here we show that the expression of stripes can also be generated by a different, two-step mode which involves regulatory interactions among the primary pair rule genes hairy (h) and runt (run). Expression of h stripes 3 and 4 is directed by a common cis-acting element that results in an initial broad band of gene expression covering three stripe equivalents. Subsequently, this expression domain is split by repression in the forthcoming interstripe region, a process mediated by a separate cis-acting element that responds to run activity. This second mode of pair rule stripe formation may have evolutionary implications.