Position-Dependent Gene Expression in Drosophila 
an escort role to position bed RNA in the right 
place in the oocyte. Another attractive hypothesis 
is that the exu protein may aid in folding the bed 
RNA into a secondary structure that is recognized 
by other factors that localize it. 
During development, positional information 
instructs genes to be expressed in different cells 
in given tissues. The Drosophila white gene is 
required for deposition of pigments in the eye of 
the fly. Normally the gene is expressed in the en- 
tire eye, leading to uniformly pigmented eyes in 
the adult. We are studying a case in which the 
white gene was inserted in a genomic location 
where its expression, rather than being uniform 
in the eye, is limited primarily to the ventral half. 
This patterned expression is due to the influence 
of DNA flanking the insertion site, since moving 
the gene to other locations in the genome does 
not have the same effect. 
Mutations were induced that eliminate the pat- 
terned expression, returning the pigmentation to 
wild type. The molecular nature of these muta- 
tions has been studied by cloning and analyzing 
the flanking DNA. These studies show that se- 
quences flanking both sides of the white gene are 
necessary for the regulation, but that a region up- 
stream of the white promoter is particularly im- 
portant for the effect. This regulatory region is 
sensitive to distance from the gene, since moving 
it closer causes more severe repression of white 
expression, leading to even less pigmented tissue 
in the eye. An intriguing aspect of the regulation 
is that it functions not only in cis to the white 
gene but also in trans on paired copies of white. 
A number of Drosophila genes show pairing- 
dependent effects on their expression, suggesting 
that promoters or labile regulatory RNAs pro- 
duced by these genes can act on a second homolo- 
gous copy of the gene. We are analyzing the 
flanking DNA and unraveling its elements in 
order to understand better this proximity-depen- 
dent type of gene regulation. 
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