Physiological activation of early-response genes is likely mediated by several of 
these second signal pathways operating simultaneously in a ceil. For instance, 
the PDGF receptor is known to activate a variety of these pathways and directly 
phosphorylate on tyrosine several proteins involved in signaling pathways 
(Williams 1989). PDGF stimulates phosphatidyl inositide turnover, thereby 
activating protein kinase C. The PDGF receptor can also bind to GAP, a 
modulator of the ras pathway, and in addition can activate c-raf (Molloy et al. 
1989; Morrison et al. 1989). Thus, a network of second signals leads to the 
activation of the appropriate set of early-response genes in any given cell type 
in response to a given stimulus. Most of those pathways have not been worked 
out in great detail and are likely to be quite complicated. This is the case even 
in the unicellular yeast Saccharomyces cerevisiae. Mating pheromones are 
extracellular peptides that regulate gene expression in yeast. A variety of 
second signaling molecules similar to those found in mammalian cells have 
been implicated in the yeast signal transduction process; these include G 
proteins and a variety of protein kinases, including one related to the CDC28 
kinase (for a review, see Herskowitz 1989). However, even with the power of 
yeast genetics, these pathways are not yet fully understood. 
TRANSCRIPTIONAL REGULATORY ELEMENTS OF lEGs 
Ultimately, a complete understanding of the induction of lEGs will have to 
come from the study of transcriptional regulatory elements of these genes 
and the factors that interact with them. Currently, the c -fos proto-oncogene is 
the best characterized of the early-response genes at the level of transcriptional 
regulation. Analysis of the upstream regulatory elements of the c -fos gene 
reveals a complex array of regulatory elements that can respond to one or 
more second messenger signals. A diagram of the c -fos promoter is shown in 
figure 1 . Upstream of the transcriptional start site is a TATA consensus box to 
which the transcription factor TFIID should bind (Buratowski et al. 1988; van 
Straaten et al. 1983). This element is characteristic of most RNA polymerase II 
promoters. Upstream of the TATA box at -65 is a Ca/CRE box, which serves at 
least three functions in the promoter. This element contributes to basal levels 
of expression (Fisch et al. 1987; Gilman et al. 1986; Runkel et al. 1991), can 
respond to increases in cAMP, and in PCI 2 cells can mediate responses to 
intracellular calcium (Berkowitz et al. 1989; Fisch et al. 1989a; Sassone- 
Corsi et al. 1988b; Sheng et al. 1988). A variety of transcription factors could 
bind to this site, but the best evidence would suggest that the CREB factor, 
which was initially isolated as a cAMP response element-binding protein, can 
likely mediate both the response to cAMP and calcium (Sassone-Corsi et al. 
1988b; Sheng et al. 1990). Recent work has shown that this factor gets 
phosphorylated by the CaM II kinase in PCI 2 cells that have undergone 
depolarization (Dash et al. 1991; Sheng et al. 1991). 
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