Transcription Control During Early Drosophila Development 
mode of recognition. In this mode amino acid 9 
fits in the major groove of DNA, in close contact 
with the base pairs recognized. 
The Paired Gene Encodes a Multifunctional 
Transcription Factor 
'-rThe product of the pair-rule gene paired (Prd) 
contains not only an HD but also other domains 
conserved in evolution that are found in develop- 
mental genes. We have shown that the 128- 
amino acid Paired domain is also DNA binding, 
making the Prd protein a bifunctional DNA-bind- 
ing transcription factor. Although both the Paired 
domain and the HD can bind to DNA indepen- 
dently, the promoter of another pair-rule gene 
has revealed sites that are bound cooperatively by 
Prd when both domains are present in the same 
protein. Extensive mutagenesis analysis of the 
protein has allowed the definition of at least four 
subfunctions in Prd. 
Transcriptional Functions of the 
Homeodomain Proteins Fushi tarazu 
and Engrailed 
In order to understand the mechanism by 
which proteins with the same DNA-binding speci- 
ficity can have different developmental func- 
tions, we have analyzed the roles of two proteins 
binding to the same target site in an in vitro tran- 
scription system. We have shown that the Fushi 
tarazu (Ftz) protein activates in vitro transcrip- 
tion in a binding site-dependent manner. This 
activation is prevented by the addition of En- 
grailed (En) , which competes with Ftz for bind- 
ing to the same sites. When more En protein is 
added, a further repression is mediated by En 
binding to the basal promoter. We have shown 
that, in this case. En acts by binding to the TATA 
box and competing with the TATA box-binding 
factor TFIID. Formation of a committed complex 
by preincubation of the promoter with TFIID 
prevents both the repression by En and its bind- 
ing to the TATA box. 
Targets of the Homeodomain 
Developmental Gene engrailed 
The HD transcription factor Engrailed belongs 
to a small set of genes required for pattern forma- 
tion within each embryonic segment and within 
imaginal discs. We are using the "enhancer trap" 
technique and a +/— screen for cDNA from undif- 
ferentiated imaginal disc cells in culture to iden- 
tify target genes affected by engrailed. In this ap- 
proach, we want to obtain the effector genes 
essential to the definition of cellular and com- 
partmental identity in discs. These genes may 
prove to be components of signaling pathways 
relevant to both early and late development. 
We have investigated both the general tran- 
scription functions carried out by the HD and 
zinc finger proteins and the particular properties 
of each gene product. Our attention is now fo- 
cused on the combinatorial effects of such factors 
in vitro and in vivo, and we expect to acquire 
insight into the mechanisms controlling the coor- 
dinate expression of developmental genes. 
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