Transcription Control During Early 
Drosophila Development 
Claude Desplan, Ph.D. — Assistant Investigator 
Dr. Desplan is also Assistant Professor and University Fellow at the Rockefeller University. He graduated 
from the Ecole Normale Superieure de Saint Cloud in Prance. He received his D.Sc. degree from the Uni- 
versity of Paris, working with Baty Moukhtar and Monique Thomasset at the INSERM on calcium regula- 
tion. He was a Maitre de Conference at the Ecole Normale Superieure de Fontenay until he moved to the 
University of California, San Francisco, to work as a postdoctoral fellow with Pat O'Farrell on the func- 
tions of homeodomain proteins during early development. 
ALL information required for the develop- 
ment of a fertilized egg into a complete or- 
ganism is contained in its own genetic material, 
contributed by both parents, and in products pro- 
vided by the mother as the egg is formed. Genetic 
studies on the fruit fly Drosophila have identi- 
fied most of the genes involved in the process of 
pattern formation. Many of these genes appear to 
encode transcription factors that contain con- 
served domains such as homeodomains or zinc 
finger domains. We believe the zygotic genome 
responds to maternal organizing factors through a 
network of transcriptional regulators to set up the 
body pattern of the embryo. Since several genes 
expressed during mammalian embryogenesis 
share homology with developmental genes of the 
fly, it is likely that the mechanisms uncovered are 
of general significance for the development of 
multicellular organisms. 
The goal of our laboratory is to understand the 
mechanisms involved in these regulatory interac- 
tions. In light of the complexity of the system, a 
productive approach is, first, to characterize the 
molecular interactions in vitro and, second, to 
design in vivo systems to test models of regula- 
tory interactions consistent with properties un- 
covered through the first approach. We are inves- 
tigating the mechanism of action of two classes of 
developmental gene products containing either a 
homeodomain (HD) or zinc finger DNA-binding 
motifs. These motifs are known to bind to DNA 
and to target regulatory proteins to their down- 
stream genes. 
Interactions Among Gap Genes 
The first zygotic genes to be expressed in the 
Drosophila embryo are the gap genes. Their role 
is to read and interpret coarse positional informa- 
tion deposited in the egg by the mother and to 
refine this information by cross-regulatory inter- 
actions. Three gap genes, Kriippel (Kr), hunch- 
back {hb), and knirps {kni), have been analyzed 
molecularly. All contain zinc finger motifs char- 
acteristic of many eukaryotic DNA-binding pro- 
teins. This homology suggests that they act as reg- 
ulators of transcription. 
A relatively simple circuit appears to control 
expression of hb. It is activated in response to the 
homeodomain-containing product of the mater- 
nal gene bicoid (bed) and is repressed by the 
neighboring gap gene Kr. The hb promoter con- 
tains DNA-binding sites for the products of the 
bed, Kr, and hb genes (Bed, Kr, and Hb), which 
have been defined in vitro. Another zygotic gene 
involved early in the development of the head is 
orthodentiele (otd) . Its product (Otd) contains 
an HD that recognizes the same sequence as the 
Bed protein and may be part of the same regula- 
tory network in the anterior part of the embryo. 
Hb, Kr, Otd, and Bed act very early, when the 
fly embryo is still a syncytium and when few 
other proteins are expressed. To study the molec- 
ular interactions among the genes of origin, we 
have introduced into transgenic flies artificial 
genes containing combinations of the binding 
sites for Kr, Hb, or Bcd/Otd in promoters driving 
the reporter gene jS-galactosidase. The patterned 
expression of the reporter gene leads to the con- 
clusion that Bed is not sufficient for expression in 
the anterior part of the embryo, despite strong 
Bed-binding sites. Addition of Hb-binding sites is 
required to obtain a promoter whose expression 
is maintained in the anterior part, mimicking that 
of hb. Our work points to a synergistic activation 
of the promoter by Bed and the maternal compo- 
nent of hb. 
The DNA-binding Specificity of the 
Homeodomain 
Over 20 genes involved in development en- 
code HD-containing proteins. The HD includes a 
domain similar to the helix-turn-helix motif of 
many prokaryotic DNA-binding proteins. Our 
analysis of the function of the HD has led us to 
describe how we can change the DNA-binding 
specificity of one HD to another, simply by re- 
placing a single amino acid at the critical position 
9 of the recognition helix. As we proposed, struc- 
tural analysis of HD-DNA complexes later showed 
that the interaction between DNA and the HD is 
not analogous to that of the prokaryotic helix- 
turn-helix proteins, but instead involves a new 
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