by nos protein may determine the profile of the hb 
protein gradient and thereby govern posterior body 
pattern. For both the bed- cad and nos- bed interac- 
tions, current experiments are in progress to define 
further the cis-acting target sites, to assay for direct 
binding, and to determine the molecular mecha- 
nism of translational repression. 
A third determinant system is responsible for spec- 
ifying the anterior and posterior ends of the body. 
This terminal system depends critically on the activ- 
ity of torso (tor), a receptor tyrosine kinase. Prior 
experiments of Drs. Jordi Casanova and Struhl have 
indicated that tor protein functions as a ubiquitous 
surface receptor that is activated in the vicinity of 
the poles in response to a localized ligand. More- 
recent experiments have suggested that when the 
receptor concentration is abnormally low, the li- 
gand is not efficiently trapped at the poles but can 
diffuse to more-central portions of the body, where 
it activates the tor receptor ectopically. The degree 
of ectopic activation observed under this unusual 
circumstance appears to depend on the dosage of a 
second gene, trunk (trk), which is critically re- 
quired for normal activation of the tor receptor. 
Drs. Casanova and Struhl have recently determined 
the sequence of the trk gene, which may encode a 
small, secreted protein. Experiments to test the role 
of the trk protein, particularly the possibility that it 
encodes the tor ligand, are in progress. 
Patterning in Cellular Systems 
Early Drosophila embryos differ from most other 
patterning systems because spatial signals arise and 
diffuse within a common cytoplasm rather than in 
populations of cells comprising organs or append- 
ages. However, few if any spatial signaling mole- 
cules have been identified in such cellular systems, 
nor are their roles well described or understood. 
One promising candidate for a diffusible factor 
that may govern cell pattern is the protein product 
of the Drosophila segment polarity gene wingless 
(wg) . It appears that wg encodes a secreted protein 
expressed in discrete subsets of cells in each embry- 
onic segment and in each imaginal disc giving rise to 
an adult appendage. In the embryo, wg activity in 
some cells is required for maintaining the expres- 
sion of the engrailed gene in neighboring cells. 
Moreover, mutations that reduce or eliminate wg 
activity have a profound effect on global cell pat- 
tern, both in embryonic segments and in the adult 
appendages. These findings have raised the possibil- 
ity that wg protein expressed by some cells may be 
responsible for organizing the pattern of surround- 
ing cells. 
Drs. Struhl and Konrad Easier have devised a new 
and general method for controlling where genes are 
expressed in developing tissues to test whether cells 
expressing wg protein can organize the pattern of 
neighboring, nonexpressing cells. The gist of this 
technique is to use a site-specific recombinase (the 
yeast protein encoded by the flp gene) to fuse the 
promoter of one gene to the coding sequence of 
another. This technology has been used to activate 
heritably the wg coding sequence in randomly posi- 
tioned cells during development. Ectopic activation 
of wg in dorsally positioned cells of the leg imaginal 
discs can reorganize the pattern of surrounding 
cells, causing dorsoventral and proximodistal pat- 
tern duplications, including the formation of super- 
numerary limbs. The expression of wg protein in a 
small subset of cells can therefore exert a profound 
influence on the developmental behavior of 
surrounding cells, indicating that these cells are 
equivalent to classical embryonic organizers. Fur- 
thermore, the nature of the duplicated patterns as 
well as the restricted positions in which they arise 
suggest the possibility that in normal leg discs ven- 
trally positioned cells provide a source for wg pro- 
tein, which spreads dorsally and functions as a mor- 
phogen to control dorsoventral pattern in the limb. 
Dr. Struhl is also Associate Professor of Genetics 
and Development and a member of the Center for 
Neurobiology and Behavior at Columbia Univer- 
sity College of Physicians and Surgeons. 
Articles 
Struhl, G., Johnston, P., and Lawrence, P.A. 1992. 
Control of Drosophila body pattern by the 
hunchback morphogen gradient. Cell 69:237- 
249. 
Wharton, R.P., and Struhl, G. 1991. RNA regula- 
tory elements mediate control of Drosophila 
body pattern by the posterior morphogen nanos. 
Cell 61 ■.955-961. 
NEUROSCIENCE 443 
