Ill intermediate filament protein. The peripherin 
gene is active at 24 h following NGF treatment, a 
time when the morphology of PCI 2 cells begins to 
change and neurites are extended. Results from the 
Ziff laboratory indicate that a peripherin promoter 
negative regulatory element (NRE) releases a pro- 
tein upon PCI 2 differentiation, and suggest that dis- 
sociation of this potential repressor, coupled with 
positive signals from other elements, activates the 
gene. In developing rat embryos, initiation of pe- 
ripherin protein synthesis coincides with the mor- 
phological differentiation of neurons and the transit 
of the cells to the postmitotic state. Thus, in devel- 
opment, peripherin induction is one constituent of 
a program of gene expression activated at the termi- 
nal stages of neuronal differentiation. 
In the terminal differentiation of neurons and 
other cell types, down-regulation of expression of 
the c-myc gene appears to be a necessary step. In 
studies supported by the National Cancer Institute, 
National Institutes of Health, the Ziff laboratory has 
investigated the c-Myc protein, a nuclear phospho- 
protein that contains structural features characteris- 
tic of a sequence-specific DNA-binding protein, in- 
cluding helix-loop-helix (HLH), leucine zipper 
(LZ), and basic region (BR) motifs. These structural 
motifs are related to the structures that enable c-Fos 
to bind to Jun and the resulting heterodimer to bind 
to DNA. 
In a search for a protein partner of Myc that could 
heterodimerize with Myc and enable it to bind to 
DNA, the laboratory cloned a mouse protein, Myn, 
which was recognized to be the murine homologue 
of the human protein Max (cloned by Elizabeth 
Blackwood and Dr. Robert Eisenman). Max and its 
murine homologue Myn are virtually identical 
~ 1 8-kDa polypeptides that contain an HLH-LZ-BR 
DNA-binding motif similar to that of the Myc pro- 
teins and that interact with c-Myc to form a specific 
DNA-binding complex. Myc protein on its own will 
not form the dimer complex requisite for binding to 
DNA. However, Myc and Max dimerize to form a 
complex that will bind to DNA. Max forms a homo- 
dimer that binds to DNA with the same sequence 
specificity as the Myc-Max heterodimer. These re- 
sults suggest that Myc-Max and Max-Max dimers 
compete for the same binding sites in DNA but after 
binding may exert different regulatory effects on the 
neighboring DNA regions. Present evidence sug- 
gests that these dimeric complexes are activators 
and repressors of transcription; however, their spe- 
cific roles are not yet established. 
DNA tumor viruses express genes that mimic 
some of the actions of growth factors and nuclear 
oncoproteins such as c-Fos and c-Myc. The adenovi- 
rus- 5 Ela gene is one such gene and encodes a cell- 
immortalizing protein that binds cellular factors, 
including the product of the retinoblastoma suscep- 
tibility locus, Rb. Rb is a tumor-suppressor protein 
that exerts a negative regulatory effect on cell 
growth. Ela stimulates cellular and viral DNA syn- 
thesis, proliferation, and transcription of adenoviral 
and cellular genes. The laboratory finds that expres- 
sion in PC 12 cells of adenovirus-5 Ela proteins re- 
sults in a loss of chromaffin cell and neuronal 
markers and causes profound changes in cell mor- 
phology. TH, peripherin, the low-affinity NGF re- 
ceptor, the trk protein, the EGF receptor, and other 
neuronal and chromaffin markers are repressed, 
while more ubiquitously expressed genes are unal- 
tered. This suggests that these differentiation- 
specific functions are sensitive to the mechanisms 
that control cell proliferation and that are perturbed 
by Ela. The region of Ela binding Rb and a 300-kDa 
cellular protein is required for these changes. The 
laboratory has found that this same region is crucial 
for activating viral early genes, including those en- 
coding DNA replication functions. The laboratory is 
currently attempting to interrelate pathways used by 
Ela, growth factors, and early-response proteins to 
control PC 12 cell proliferative and differentiation 
functions. 
Dr. Ziff is also Professor of Biochemistry at New 
York University Medical Center. 
Articles 
Metz, R., and Ziff, E. 1991. cAMP stimulates the 
C/EBP-related transcription factor rNFIL-6 to 
trans-XocdXe to the nucleus and induce c-fos tran- 
scription. Genes Dev 5:1154-1766. 
Metz, R., and Ziff, E. 1991. The helix-loop-helix 
protein rEl 2 and the C/EBP-related factor rNFIL-6 
bind to neighboring sites within the c-fos serum 
response element. Oncogene 6:2165-2178. 
Ross, D., and Ziff, E. 1992. Defective synthesis of 
early region 4 mRNAs during abortive adenovirus 
infections in monkey cells. / Virol 66:3110- 
3117. 
Thompson, M.A., Lee, E., Lawe, D., Gizang-Ginsberg, 
E., and Ziff, E.B. 1992. Nerve growth factor- 
induced derepression of peripherin gene expres- 
sion is associated with alterations in proteins bind- 
ing to a negative regulatory element. Mol Cell 
Biol 12:2501-2513. 
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