precisely restricted temporal and spatial patterns of 
gene expression. In this regard, the POU domain 
family of transcription factors resemble the devel- 
opmental patterns of the hierarchy of regulatory 
genes that are sequentially activated during Dro- 
sophila development. The correlation between ex- 
pression of these genes and cortical and sensory 
neuronal development suggests possible roles for 
these factors in determining mature neuronal phe- 
notypes. The potential functions of POU domain 
proteins in activating specific patterns of gene ex- 
pression characteristic of mature neurons are 
under investigation. 
III. Hormonal Regulation of Gene Expression. 
Growth and regulation of eukaryotes are under 
complex control and are regulated by diverse fami- 
lies of peptides, collectively referred to as growth 
factors. The epidermal growth factor (EGF) recep- 
tor is a 170 kDa transmembrane glycoprotein with 
intrinsic protein tyrosine kinase activity. The recep- 
tor autophosphorylates multiple sites on its car- 
boxyl terminus and exhibits heterologous regula- 
tion due to phosphorylation by protein kinase C. 
Binding of ligand to the EGF receptor initiates a se- 
ries of immediate, rapid, and delayed effects that 
culminate in DNA replication and cell division, in- 
cluding activation of the intrinsic protein kinase ac- 
tivity, increase in intracellular calcium, alterations 
in intracellular pH, receptor internalization and 
downregulatory activation of specific gene tran- 
scription, and, ultimately, DNA replication and mi- 
tosis. The cloning of the EGF receptor has permit- 
ted definition of a series of distinct domains, 
including a carboxyl-terminal inhibitory domain, a 
distinct region containing an 18-amino acid, acidic 
helix bounded by turns, that is required for cou- 
pling receptor to effective internalization and cal- 
cium regulatory mechanisms. The identification of 
this putative domain and the creation of a series of 
mutants eliminating its activity permit a critical test 
of various models of receptor activation increases of 
intracellular calcium levels. Because a point muta- 
tion of the ATP-binding site that eliminates all in- 
trinsic protein kinase activity abolishes all of the 
known effects of EGF (including receptor internal- 
ization), internalization of the receptor itself could 
represent the critical signaling event. An analysis of 
a series of mutant receptors has shown that a ki- 
nase-active, internalization-defective receptor can 
transmit signals for regulated gene expression and 
cell growth, supporting the hypothesis that phos- 
phorylation of specific substrates by this mem- 
brane-bound, uninternalized receptor signals the 
mitotic effects of EGF. Receptor downregulation 
serves a critical function in preventing signal atten- 
uation, resulting in morphological transformation. 
Pit-1 cis-active elements transfer regulation by 
EGF, thyrotropin-releasing hormone (TRH), and 
phorbol esters; additional regulatory elements in 
the prolactin and growth hormone genes have 
been identified. A second pathway of regulation by 
calcium, involving type II calcium, calmodulin 
(CaM)-dependent protein kinase, has been identi- 
fied based on cloning of the brain-specific CaM pro- 
tein kinase a and construction of a constitutive vari- 
ant. 
The cloning of ligand-dependent transcription 
factors and identification of estrogen and T^ re- 
sponse elements in the rat prolactin and growth 
hormone genes, respectively, have permitted defini- 
tion of receptor domains and a study of the molec- 
ular basis for positive and negative regulation of 
gene transcription. Specific classes of T^ receptors 
can bind to estrogen response elements with high 
affinity, but in a transcriptionally inactive form, re- 
sulting in a net decrease in gene expression. These 
data reveal that only a subset of T^ -binding 
elements function as T^ response elements. 
Heterodimers between members of the nuclear re- 
ceptor superfamily (T^ receptor and retinoic acid 
receptor) have been identified and shown to be ca- 
pable of exerting positive and negative effects on 
gene transcription, dependent on the sequence of 
the cis-active sequences to which they bind. The p 
form of the T^ receptor forms a heterodimer with 
the a form of the human retinoic acid receptor, 
based on critical carboxyl-terminal sequences, ex- 
erting opposite transcriptional effects on different 
Tj response elements. 
IV Developmental Regulation at a Post-transcrip- 
tional Level. 
Based on an analysis of the rat and human calci- 
tonin/CGRP (calcitonin gene-related peptide) 
genes, alternative UNA processing has been demon- 
strated to represent an important developmental 
strategy used in the neuroendocrine system to dic- 
tate a tissue-specific pattern of polypeptide product 
production. An analysis of the molecular mecha- 
nisms responsible for generating such restricted 
patterns of gene expression has been initiated, to 
provide general insights into the molecular strate- 
gies critical for development and function of the 
Continued 
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