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molecule nitric oxide (NO), and we will discuss these findings in a later
section (
Caceres et al., 2011
).
As briefly alluded to above, many components of the ecdysone regula-
tory hierarchy are—like EcR and USP—members of the nuclear receptor
superfamily. These include
Drosophila
hormone receptor 3 (DHR3),
DHR4, DHR39, E75, E78, and FTZ-F1 (
King-Jones & Thummel,
2005; Woodard, Baehrecke, & Thummel, 1994
). Below, we will briefly
review the roles of DHR3, DHR4, and FTZ-F1 in the context of early
metamorphic stages.
5. THE ECDYSONE HIERARCHY III: EARLY-LATE
RESPONSE GENES AND
b
ftz-f1
Early-late genes can be operationally defined as genes that require both
the 20E-bound EcR/USP heterodimer and an early gene product for max-
imal transcriptional induction (
Fig. 2.1
B). This is typically shown in organ
culture assays using protein synthesis inhibitors to block the translation of
early gene mRNAs. Two early-late genes with very similar temporal expres-
sion profiles are the nuclear receptor genes
DHR3
and
DHR4
. DHR3 is
orthologous to the vertebrate retinoid-related orphan receptor (ROR),
while DHR4 is represented by the germ-cell nuclear factor (GCNF) in ver-
tebrates. DHR3 and DHR4 expression profiles show a peak at the begin-
ning of prepupal stage, when the expression of early genes such as
BR-C
,
E74A
, and
E75A
is receding, and
b
ftz-f1
expression is about to be induced.
Both DHR3 and DHR4 are sufficient to repress the early genes and are re-
quired for maximal
b
ftz-f1
expression in mid-prepupae (
King-Jones, Charles,
Lam, & Thummel, 2005; Lam, Jiang, & Thummel, 1997
), strongly suggesting
that these two factors act in concert to regulate the early genes and
b
ftz-f1
.In-
terestingly,
DHR4
mutants display precocious wandering behavior followed
by premature onset of metamorphosis, resulting in a small body size due to a
shortened feeding period, a phenotype not observed in any other mutants as-
sociated with the ecdysone hierarchy (
King-Jones et al., 2005
). This peculiar
phenotypewas eventually tracked to a role for DHR4 in the prothoracic gland,
which we will discuss in the next section.
The
Drosophila ftz-f1
gene encodes yet another nuclear receptor acting in
the ecdysone cascade and is orthologous to vertebrate steroidogenic factor 1
(SF-1). Two protein isoforms have been described,
a
FTZ-F1 and
b
FTZ-F1
(
Lavorgna, Ueda, Clos, & Wu, 1991; Ueda, Sonoda, Brown, Scott, & Wu,
1990
). While
a
FTZ-F1 is maternally supplied and has critical roles in
