Bicoid determines sharp and precise target gene expression in the Drosophila embryo.
BACKGROUND
The activity of the Bicoid (Bcd) transcription factor is a useful example of how quantitative information contained in a smooth morphogen gradient is transformed into discrete and precise patterns of target gene expression. There are two distinct and important aspects to this process: the "sharpening" of the posterior borders of the expression domains and the "precision" of where the target genes are expressed along the length of the embryo as the syncytial embryo begins to cellularize. Although the sharpening phenomenon was observed over a decade ago, it is still poorly understood.


RESULTS
Here, we show that a Bcd reporter gene containing binding sites only for Bcd is expressed, like natural targets of Bcd, in a precise domain with a sharp boundary. Analysis of embryos expressing deleted forms of Bcd indicates that the sharpness of the Bcd target gene hunchback's expression involves the glutamine-rich and C-terminal activation domains of Bcd. Furthermore, several artificial Gal4-derived transcription factors expressed as gradients in the embryo share Bcd's ability to drive precise target gene expression with sharp boundaries.


CONCLUSIONS
Thus, contrary to recent reports proposing that the Bcd gradient is not sufficient to establish precise positional information, we show that Bcd drives precise and sharp expression of its target genes through a process that depends exclusively on its ability to activate transcription.
Crauk
Olivier
O
Laboratory of Nuclear Dynamics and Genome Plasticity, CNRS/UMR 218, Institut Curie, Paris, France.
Dostatni
Nathalie
N
D000818:Animals; D019521:Body Patterning / Q000502:physiology*; D004330:Drosophila / Q000196:embryology* / Q000378:metabolism; D029721:Drosophila Proteins / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D018398:Homeodomain Proteins / Q000378:metabolism*; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008954:Models, Biological; D016601:RNA-Binding Proteins / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015534:Trans-Activators / Q000378:metabolism*
D029721:Drosophila Proteins; D018398:Homeodomain Proteins; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; C056478:bcd protein, Drosophila; C069927:stau protein, Drosophila