In contrast to the 5' deletion constructs, neither of 
the intronic deletions is capable of completely re- 
scuing the pb mutant phenotype. The region in in- 
tron 2 thus appears to be essential for pb function, 
and removing sequences within it results in aberrant 
adult mouthparts characteristic of pb hypomorphic 
or amorphic mutants. These sequences do not direct 
lacZ in a pb pattern in combination with a heterolo- 
gous promoter (hsp70) but are capable of regu- 
lating lacZ in a pb pattern when located in a re- 
porter gene containing the pb promoter. Thus the 
conserved region appears to require the pb pro- 
moter to function properly. 
At present the simplest model predicts that the 
second intron enhancer region coupled with the pb 
promoter is capable of directing pb protein in a 
broad region both anterior and posterior to the wild- 
type domain. Refinement into the normal pb pattern 
is achieved by the added action of the upstream re- 
pressor elements defined above. (This work is sup- 
ported by a grant from the National Institutes of 
Health.) 
The labial locus 
The cis-acting elements of the lab locus have 
been defined in a similar manner to that described 
for pb and Scr. The protein products of this gene are 
normally accumulated in the ectoderm of the inter- 
calary segment and dorsal ridge, the tritocerebral 
ganglion, and the endoderm of the anterior and pos- 
terior midgut. A series of reporter and minigene con- 
structs have demonstrated that each of these spatial 
domains of expression is separately controlled by a 
unique set of cis-acting regulatory elements. Fur- 
thermore these elements interact with a distinctive 
set of trans-acting loci. 
Initiation of expression in the intercalary segment 
requires only 750 bp of upstream sequence and the 
lab promoter. This expression is triggered by the 
action of the buttonhead and empty spiracles 
genes. Maintenance of lab expression, however, re- 
quires the presence of protein, i.e., lab is autog- 
enously regulated. This feedback loop also requires 
an additional 3 .0-kb fragment of upstream DNA and 
only operates in the ectoderm of the intercalary seg- 
ment. The ectoderm of the dorsal ridge is controlled 
by a separate enhancer element that is located in the 
first intron of the gene and does not require the lab 
promoter. Finally, the expression of lab in the endo- 
derm is controlled by two separate enhancer ele- 
ments and two difl'erent trans-acting members of the 
anterior-posterior segmentation pathway. 
The anterior midgut expression requires a 500-bp 
fragment upstream of the start of transcription and 
the action of the huckebein locus. The posterior 
midgut accumulation is mediated by a 500-bp frag- 
ment in the first intron and requires the presence of 
the forkhead gene. Thus, unlike the pb and Scr 
genes, the regulation of lab appears to be entirely 
positive. (This work is supported by a grant from the 
National Institutes of Health.) 
Dr. Kaufman is also Professor of Genetics in the 
Department of Biology at Indiana University, 
Bloomington, and Adjunct Professor of Medical 
Genetics in the Department of Medical Genetics at 
Indiana University Medical Center. 
Articles 
Chouinard, S., and Kaufman, T.C. 1991 . Control of 
expression of the homeotic labial (lab) locus of 
Drosophila melanogaster-. evidence for both pos- 
itive and negative autogenous regulation. Devel- 
opment 113:1267-1280. 
Cribbs, D.L., Pultz, M. A., Johnson, D., MazzuUa, M., 
and Kaufman, T.C. 1992. Structural complexity 
and evolutionary conservation of the Drosoph- 
ila homeotic gene proboscipedia. EMBO f 
11:1437-1449. 
Heuer, J.G., and Kaufman, T.C. 1992. Homeotic 
genes have specific functional roles in the estab- 
lishment of the Drosophila embryonic peripheral 
nervous system. Development 1 15:35-47. 
Tamkun, J.W., Deuring, R., Scott, M.P., Kissinger, 
M., Pattatucci, A.M., Kaufman, T.C, and Kenni- 
son, J.A. 1992. brahma: a regulator of Drosoph- 
ila homeotic genes structurally related to the 
yeast transcriptional activator SNF2/SW12. Cell 
68:561-572. 
GENETICS 209 
