TRANSCRIPTIONAL CONTROL IN EARJY DROSOPHILA DEVELOPMENT 
CiAUDE Despian, Ph.D., Assistant Investigator 
The information required for the development 
of a fertilized egg into a complete organism is en- 
coded in the genetic material of the zygote pro- 
vided by both parents, as well as in products 
loaded by the mother during the formation of the 
egg. The availability of genetics in the fruit fly Dro- 
sophila has allowed the identification of many of 
the genes involved in the process of development. 
Many genes appear to encode transcription factors 
and to share common protein domains. Through a 
network of transcriptional regulators, the zygotic 
genome responds to maternal organizing factors to 
set up the body pattern of the embryo. The goal of 
the current research in Dr. Desplan's laboratory is 
to understand the mechanisms involved in these 
regulatory interactions. The functions of two 
classes of developmental gene products that con- 
tain either a homeodomain (HD) or zinc finger 
DNA-binding motif are being investigated. Of partic- 
ular interest are the interactions governing DNA- 
binding specificity of these proteins and their ac- 
tion in controlling transcription of their target 
genes. Since several genes expressed during 
embryogenesis in mammals share homology with 
the Drosophila developmental genes, it is likely 
that the mechanisms uncovered are also of sig- 
nificance for the development of multicellular or- 
ganisms. 
I. 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 information 
deposited in the egg by the mother and to refine 
this information by cross-regulatory interactions. 
Three gap genes that have been analyzed molecu- 
larly contain zinc finger motifs characteristic of 
many eukaryotic DNA-binding proteins. This ho- 
mology suggests that they might act as regulators of 
transcription. A relatively simple circuit appears to 
control expression of one of the gap genes, hunch- 
back (hb). Jessica Treisman has analyzed how the 
products of the Kriippel {Kr) and hb gap genes are 
involved in this pathway. The Kr protein produced 
in Escherichia coli is capable of binding to the se- 
quence AAGGGGTTAA and to related DNA se- 
quences, while the Hb protein recognizes the con- 
sensus sequence ACNCAAAAAANTA and related 
sites. Synthetic repeats of these consensus se- 
quences are also recognized, showing that they are 
sufficient for binding. Two Kr-binding sites that 
could mediate the repression of the proximal pro- 
moter of the hb gene by Kr have been identified. In 
vivo this promoter drives hb expression in an ante- 
rior stripe, and in the absence of Kr the posterior 
border of this domain expands into the Kr domain. 
Binding sites for the Hb protein are also present 
upstream of both hb promoters. In vivo these sites 
may allow hb to influence its own expression. Mar- 
cia Simpson is now investigating the in vivo func- 
tion of these binding sites for the correct patterned 
expression of hb. 
The relative simplicity of the control of hb by a 
positive activator, the homeodomain-containing 
protein product of the maternal gene bicoid, and 
the action of a negative factor, Kr, will allow re- 
searchers to understand how interactions of these 
proteins with the transcriptional machinery occur 
in the early syncytial embryo. For this purpose, 
Treisman has constructed P-element vectors con- 
taining combinations of synthetic binding sites for 
Bed, Kr, and Hb behind promoters not expressed 
(the heat-shock basal promoter) or expressed at 
high level in the early embryo (the Serendipity pro- 
moter). She is looking at activation and repression 
by these target sites in embryos and in Schneider 
cell culture. 
II. A Single Amino Acid Determines the Specificity 
of Homeodomain Proteins. 
Over 20 genes involved in development encode 
homeodomain (HD)-containing proteins. The HD 
includes a domain similar to the helix-turn-helix 
motif present in many prokaryotic DNA-binding 
proteins. The HD proteins can be classified on the 
basis of the sequence of their presumed recogni- 
tion helix. By performing an extensive in vitro mu- 
tagenesis of several homeodomain proteins, Jessica 
Treisman, Pierre Gonczy, and Malini Vashishtha 
have been able to define a very simple rule that de- 
termines the specificity of the several classes of 
homeodomains. The last amino acid of the recogni- 
tion helix appears to be the necessary and sufficient 
determinant of the specificity of a homeodomain 
protein. In particular, the specificity of Paired has 
been changed to that of another pair-rule gene 
product, Fushi tarazu (Ftz), by replacing a serine by 
a glutamine at position 9 of the recognition helix. 
Continued 
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