Genetic Control of Morphogenesis 
pable of this activity. We are currently testing this 
hypothesis in transgenic animals. 
The proboscipedia Gene 
As is the case for the lab locus, we have con- 
structed a minigene of the pb transcription unit, 
returned it to flies, and alfected a rescue of the 
adult mouthpart-to-leg transformation. However, 
unlike lab we were unable to identify sequences 
upstream of the start of transcription that were 
sufficient to drive expression of a |8-galactosidase 
reporter gene. We have now found the fragments 
necessary for a pb pattern of expression in the 
major intron of the gene. Specifically, a 2-kb frag- 
ment from the intron in combination with 7 kb of 
upstream sequence is sufficient to drive reporter 
expression in a pb pattern. The intronic regula- 
tory elements are evolutionarily highly con- 
served, and identical DNA sequences can be 
found in the same position in the pb loci of two 
other distantly related Drosophila species. We 
are currently defining the extent of the pb pro- 
moter through a deletion analysis of the appro- 
priate sequences in both the minigene and re- 
porter constructs. Moreover, we are testing the 
ability of the homologous sequences from the 
two related flies to drive pb expression in D. 
melanogaster. 
The Sex combs reduced Gene 
We have shown that Scr locus expression in the 
adult is regulated by a transvection-like mecha- 
nism. Normally 5cr protein is accumulated in the 
labial and first thoracic imaginal discs. Transvec- 
tion mutants also express the gene in the second 
and third thoracic segments. This deregulation 
occurs by a misexpression of the copy of the gene 
held in trans to the chromosome that bears the 
mutant allele. To locate the sequences that re- 
spond to the transvection signal, we have cloned 
portions of the Scr regulatory DNA into appro- 
priate reporter constructs. These are now in flies, 
and we are in the process of determining their 
expression patterns in both normal and transvec- 
tion-inducing mutant backgrounds. 
Regulated Ectopic Expression of the 
ANT-C Homeotics 
We have built constructs in which each of the 
ANT-C homeotic loci are driven by the hsp70 
promoter and have determined the effects of 
global expression of each of these genes on mor- 
phology and the expression of the other home- 
otic loci. Now we are using a two-element system 
developed in the laboratory of Norbert Perrimon 
(HHMI, Harvard Medical School) to fine-tune this 
approach. The promoter elements of lab and pb 
have been cloned in front of the yeast GAL4 struc- 
tural gene. Additionally cDNAs and genomic frag- 
ments from all five ANT-C homeotics have been 
cloned downstream of a promoter that is regu- 
lated by the GAL4 protein. These constructs will 
be transformed separately into flies. It will then 
be possible to express each of the homeotic struc- 
tural genes in the domains of the lab and pb pro- 
moters by crossing flies containing either of the 
promoter-GAL4 constructs to animals carrying 
each of the receptor- homeotic chimeras. These 
constructs and crosses will allow us to determine 
if the precise ectopic expression of one homeotic 
gene has any effect (interference or ameliora- 
tion) on the locus normally expressed in that spa- 
tiotemporal environment. 
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