Neither the homeodomain nor the activation domain of Bicoid is specifically required for its down-regulation by the Torso receptor tyrosine kinase cascade.
Bicoid (Bcd) is a maternal morphogen responsible for patterning the head and thorax of the Drosophila embryo. Correct specification of head structure, however, requires the activity of the Torso receptor tyrosine kinase cascade, which also represses expression of Bcd targets at the most anterior tip of the embryo. Here, we investigate the role of both the homeodomain (HD) and the activation domain of Bcd in the anterior repression of its targets. When a Bcd mutant protein whose HD has been replaced by the Gal4 DNA-binding domain is expressed in early embryos, a reporter gene driven by Gal4 DNA-binding sites is first activated in an anterior domain and then repressed from the anterior pole. The down-regulation of Bcd-Gal4 activity requires torso function but does not depend on endogenous bcd activity, indicating that the Bcd protein alone and none of its targets is required to mediate the effect of torso. Functional analysis of a chimeric protein, whose activation domain has been replaced by a generic activation domain, indicates that the activation domain of Bcd is also not specifically required for its down-regulation by Torso. We propose that Torso does not affect the ability of Bcd to bind DNA, but instead directs modification of Bcd or of a potential Bcd co-factor, which renders the Bcd protein unable to activate transcription.
Bellaïche
Y
Y
Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10021, USA.
Bandyopadhyay
R
R
Desplan
C
C
Dostatni
N
N
D000818:Animals; D001667:Binding, Competitive; D019521:Body Patterning; D004268:DNA-Binding Proteins / Q000378:metabolism; D015536:Down-Regulation; D029721:Drosophila Proteins; D004331:Drosophila melanogaster / Q000196:embryology* / Q000235:genetics; D005260:Female; D018507:Gene Expression Regulation, Developmental; D006257:Head / Q000196:embryology; D018398:Homeodomain Proteins; D007301:Insect Hormones / Q000737:chemistry* / Q000502:physiology; D020794:Receptor Protein-Tyrosine Kinases / Q000502:physiology*; D011993:Recombinant Fusion Proteins; D015398:Signal Transduction; D013329:Structure-Activity Relationship; D015534:Trans-Activators; D014158:Transcription, Genetic
D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D018398:Homeodomain Proteins; D007301:Insect Hormones; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; C056478:bcd protein, Drosophila; D020794:Receptor Protein-Tyrosine Kinases; C080148:tor protein, Drosophila