CONTROL OF MORPHOGENESIS BY THE DROSOPHILA HOMEOTIC GENE UITRABITHORAX 
Philip A. Beachy, Ph.D., Assistant Investigator 
The body plan of Drosophila, like that of many 
metazoa, is characterized by a series of homologous 
segments, or metameres, that bear structures spec- 
ialized for feeding, locomotion, reproduction, and 
other functions. This anatomical format results 
from the sequential deployment during embryonic 
development of several hierarchically arranged 
groups of genes. The products of many of the genes 
within this hierarchy have been implicated in tran- 
scriptional control, either through direct biochemi- 
cal studies or by the presence of certain structural 
motifs associated with DNA-binding activity. Little is 
known, however, of the mechanisms by which reg- 
ulatory instructions issued from within the nucleus 
control morphogenetic pathways leading to the di- 
verse morphologies of individual segments. Dr. 
Beachy's laboratory is attempting to identify 
morphogenetic genes in Drosophila that are targets 
for the products of genes within regulatory levels of 
the hierarchy and to understand the mechanisms 
that govern expression of these target genes. 
Research is focused on the Drosophila homeotic 
gene Ubx {Ultrabithorax), which, together with 
half a dozen other homeotic genes, occupies the 
lowest and latest-acting tier of the pattern forma- 
tion hierarchy. The homeotic genes act within a re- 
petitive segmented framework established by genes 
within higher tiers of the hierarchy to produce the 
specialized structures that distinguish the seg- 
ments. Ubx is primarily responsible for the distin- 
guishing features of a contiguous region including 
parts of two thoracic segments and one abdominal 
segment. Past work has demonstrated that Ubx en- 
codes a family of closely related nuclear proteins 
with sequence-specific DNA-binding properties; 
these proteins are capable of modulating transcrip- 
tion in vivo, and transcriptional control is likely to 
be the primary mechanism of Ubx and other 
homeotic gene action during development. With 
this as a starting point. Dr. Beachy's laboratory is 
pursuing several strategies for the identification of 
target genes. 
I. Isolation of Tightly Bound DNA Sequences. 
The effects of Ubx regulation are likely mediated 
through the interactions of Ubx proteins with spe- 
cific DNA sequences located near target genes. One 
approach to the isolation of target genes therefore 
would utilize Ubx protein as a reagent to identify 
and isolate genomic DNA sequences bound with 
high affinity. A large-scale procedure based on spe- 
cific-sequence DNA affinity chromatography has 
been developed that yields milligram quantities of 
nearly homogeneous Ubx protein from an overpro- 
ducing Escherichia coli strain. Conditions for nitro- 
cellulose filter-binding and gel-retention methods 
have been developed that should permit the isola- 
tion of specific protein complexes with Drosophila 
genomic DNA fragments. 
II. Isolation of Genes and Promoters Responsive to 
Ubx Expression. 
This laboratory has developed several clonally 
derived cell lines in which Ubx expression is under 
control of the Drosophila metallothionein pro- 
moter. Conditions have been established in which 
Ubx proteins are induced and maintained at intra- 
cellular levels similar to those in embryos. It should 
now be possible to isolate sequences differentially 
expressed in the induced and uninduced state; 
these sequences will be tested for proximity to Ubx 
protein-binding sites, perhaps by cross-hybridiza- 
tion to isolated genomic sequences. 
To identify promoters that respond to Ubx ex- 
pression in embryos, several hundred strains, each 
containing an independent insertion of a P ele- 
ment-based "enhancer sniffer," have been exam- 
ined by histochemical staining of embryos for (3- 
galactosidase activity. The enhancer sniffers contain 
the E. coli (B-galactosidase gene under control of a 
weak promoter, whose activity is strongly influ- 
enced by genomic context. Many of the strains dis- 
play tissue- or cell-specific staining patterns; of par- 
ticular interest is a subset of these that show 
cell-specific patterns with clear segmental differ- 
ences, because these insertions may identify regula- 
tory sequences responsive to homeotic genes. 
These strains will be examined for altered expres- 
sion patterns in genetic backgrounds where Ubx 
protein expression is altered. For strains that merit 
further study, Drosophila genomic DNA flanking 
the site of insertion can be isolated by marker res- 
cue, using an antibiotic resistance gene and a plas- 
mid origin within the enhancer sniffer. 
III. DNA Looping and the Mechanism of Ubx Action. 
Protein-mediated DNA looping has been sug- 
gested as a mechanism by which eukaryotic en- 
hancers and certain regulatory sequences in pro- 
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