Fd. Karim et al., THE DROSOPHILA BROAD-COMPLEX PLAYS A KEY ROLE IN CONTROLLING ECDYSONE-REGULATED GENE-EXPRESSION AT THE ONSET OF METAMORPHOSIS, Development, 118(3), 1993, pp. 977-988
During Drosophila third instar larval development, one or more pulses
of the steroid hormone ecdysone activate three temporally distinct set
s of genes in the salivary glands, represented by puffs in the polyten
e chromosomes. The intermolt genes are induced first, in mid-third ins
tar larvae; these genes encode a protein glue used by the animal to ad
here itself to a solid substrate for metamorphosis. The intermolt gene
s are repressed at puparium formation as a high titer ecdysone pulse d
irectly induces a small set of early regulatory genes. The early genes
both repress their own expression and activate more than 100 late sec
ondary-response genes. The Broad-Complex (BR-C) is an early ecdysone-i
nducible gene that encodes a family of DNA binding proteins defined by
at least three lethal complementation groups: br, rbp, and l(1)2Bc. W
e have found that the BR-C is critical for the appropriate regulation
of all three classes of ecdysone-inducible genes. Both rbp and l(1)2Bc
are required for glue gene induction in mid-third instar larvae. In a
ddition, the l(1)2Bc function is required for glue gene repression in
prepupae; in l(1)2Bc mutants the glue genes are re-induced by the late
prepupal ecdysone pulse, recapitulating a mid-third instar regulatory
response at an inappropriate stage in development. The l(1)2Bc functi
on is also required for the complete ecdysone induction of some early
mRNAs (E74A, E75A, and BR-C) and efficient repression of most early mR
NAs in prepupae. Like the intermolt secondary-response genes, the late
secondary-response genes are absolutely dependent on rbp for their in
duction. An effect of l(1)2Bc mutations on late gene activity can also
be detected, but is most likely a secondary consequence of the submax
imal ecdysone-induction of a subset of early regulatory products. Our
results indicate that the BR-C plays a key role in dictating the stage
-specificity or the ecdysone response. In addition, the ecdysone-recep
tor protein complex alone is not sufficient for appropriate induction
of the early primary-response genes, but requires the prior expression
of BR-C proteins. These studies define the BR-C as a kev regulator of
gene activity at the onset of metamorphosis in Drosophila.