CONTROL OF GENE EXPRESSION DURING THE CELL CYCLE AND 
IN THE DEVELOPING MAMMALLSJV CEREBELLUM 
Nathaniel Heintz, Ph.D., Associate Investigator 
The studies pursued in Dr. Heintz' s laboratory 
are guided by the thought that most interesting bi- 
ological transitions, whether within the life of a sin- 
gle cell or during the development of a complex tis- 
sue, are accompanied by changes in the expression 
of specific genes. Thus knowledge of the molecular 
events that result in the activation of these genes in 
response to such a transition can lead to a detailed 
understanding of the transition. Dr. Heintz is utiliz- 
ing this approach to examine specific transitions 
that occur in two different biological contexts: the 
mammalian cell division cycle and the developing 
mouse cerebellum. 
I. Control of Gene Expression During the Cell Cycle. 
Approximately 20 years ago it was demonstrated 
that histones are synthesized at significant rates 
only during the S phase of the cell cycle. Early stud- 
ies in this and other laboratories established that 
these proteins are encoded by a small supergene 
family and that transcriptional control is important 
for the increased expression of these proteins dur- 
ing the transition from Gl to S phase. Furthermore, 
Dr. Heintz and his colleagues demonstrated that 
transcriptional regulation of histone gene expres- 
sion is due to subtype-specific transcription factors 
that bind to highly conserved sequence elements 
shared by individual genes coding for a particular 
histone subtype and that this type of regulation 
could be reproduced in vitro. These observations 
led to a simple model for coordinate induction of 
histone gene expression, involving activation of dis- 
tinct transcription factors by a common mechanism 
that becomes active during the transition from Gl 
to S phase. 
During the past year. Dr. Heintz's laboratory has 
continued analysis of five histone gene transcrip- 
tion factors (H4TF1, H4TF2, OTFl, HlTFl, H1TF2) 
and their role in cell cycle regulation of transcrip- 
tion. In particular, in-depth analysis of histone HI 
transcription in extracts from homogenous popula- 
tions of Gl and S phase cells prepared by centrifu- 
gal elutriation has confirmed and extended the 
model that histone gene regulation is mediated by 
subtype-specific transcription factors. In this case, S 
phase induction is achieved through the agency of 
two distinct Hl-specific transcription factors, 
HlTFl and H1TF2. Quantitative DNA-binding as- 
says indicate that, in contrast to HlTFl and the 
H2b factor OTFl, H1TF2 DNA binding is elevat- 
ed in S phase HeLa cells. Thus, although all three 
of these factors directly participate in coordinate 
activation of histone gene expression upon entry 
into S phase, their biochemical response to this 
transition is not uniform. Current efforts focus on 
chemical characterization of each of the histone- 
specific transcription factors and generation of 
monospecific antibodies to them. Relatively large 
quantities of H4TF2, OTFl, HlTFl, and H1TF2 
have been purified and are being utilized for these 
purposes. 
Dr. Heintz's laboratory has also begun to investi- 
gate whether the same mechanisms alluded to 
above might regulate other cellular genes whose 
expression is temporally controlled during the cell 
cycle. In particular, experiments have been initiated 
to dissect the human thymidine kinase (TK) pro- 
moter (in collaboration with Dr. S. Conrad) to iden- 
tify DNA sequences and protein factors important 
for their temporal regulation. Several novel factors 
interacting with the TK promoter have been identi- 
fied, although a role for these proteins in cell cycle 
regulation has not been established. The observa- 
tion that both of the histone HI cell cycle regula- 
tory factors specifically interact with the TK pro- 
moter suggests a possible common mechanism for 
regulation of TK and HI gene expression. 
II. Control of Gene Expression in the Mammalian 
Cerebellum. 
The mammalian cerebellum is a complex and 
highly stereotyped structure in which major pattern 
formation and functional organization occur post- 
natally. It is therefore amenable to study, and its 
development has been described in detail at the 
histological level. An extensive literature has docu- 
mented the importance of cell-cell interactions in 
the generation and maintenance of normal cerebel- 
lar architecture. Furthermore, there are at least 
eight recessive mutations in inbred mouse strains 
that perturb cerebellar structure and function. 
Dr. Heintz's laboratory has initiated several ap- 
proaches toward the isolation of genes that are ei- 
ther required for or respond to specific transitions 
that occur during the development of the mouse 
cerebellum. To identify genes that are required for 
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