the development of cerebellar neurons are marked 
by expression of these genes. Dr. Heintz and his col- 
leagues are interested in identifying the molecular 
mechanisms that regulate such stage-specific ex- 
pression and in understanding the functions of these 
novel developmentally regulated proteins in the 
mammalian cerebellum. 
Control of Macromolecular Synthesis 
During the Cell Cycle 
In studies supported principally by the National 
Institutes of Health, Dr. Heintz's laboratory has es- 
tablished that transcriptional induction of histone 
gene expression during the S phase of the mamma- 
lian cell cycle involves coordinate activation of a set 
of transcription factors that interact with subtype- 
specific consensus sequences within the histone 
gene promoters. Thus each of the approximately 
dozen genes encoding histone H2b contains a highly 
conserved element that interacts with the transcrip- 
tion factor Oct- 1 to promote S-phase-specific tran- 
scription. Distinct but functionally similar subtype- 
specific consensus elements and their cognate 
transcription factors are also important for cell 
cycle-control of histone HI and H4 genes. The tran- 
scription factors (Oct-1, HlTFl, H1TF2, and 
H4TF2) interacting with these cell cycle-control 
sequences have been isolated and characterized, 
and their behavior during the cell cycle is now be- 
ing investigated. 
Recent investigations have established that the 
H2b transcription factor Oct-1 undergoes a com- 
plex program of phosphorylation during the cell cy- 
cle that correlates with changes in its functional ac- 
tivity. One aspect of this program is the mitotic 
hyperphosphorylation of Oct-1, which leads to its 
functional inactivation at this time in the cell cycle. 
Thus phosphorylation within the Oct-1 DNA- 
binding domain occurs in vivo and can be repro- 
duced in vitro using purified protein kinase A 
(PKA). These observations demonstrate that post- 
translational modifications play an important role in 
regulating transcription factor activity during mito- 
sis, and implicate PKA in the regulation of transcrip- 
tion by the POU family of DNA-binding proteins. 
Present efforts are directed toward identifying 
both the enzymes responsible for additional modifi- 
cations of Oct- 1 during the cell cycle and their func- 
tional consequences. To determine whether post- 
translational modifications also modulate the 
activity of other histone gene transcription factors 
implicated in cell cycle control, the laboratory has 
extended these studies to H1TF2, a transcription 
factor that binds to the histone HI S-phase regula- 
tory sequence. This factor is a novel heterodimeric 
complex that is entirely distinct biochemically from 
Oct- 1 , yet the results indicate that it is also regulated 
late in the cell cycle by phosphorylation. This pro- 
vides the first biochemical evidence that coordinate 
control of histone gene transcription during the cell 
cycle appears to be effected through molecular 
mechanisms that modulate functionally related but 
biochemically distinct factors. 
Of particular interest is the possibility that either 
these transcription factors or the mechanisms regu- 
lating their activity might also be utilized to control 
the initiation of DNA synthesis during S phase. In 
collaboration with Dr. Lisa Dailey (Rockefeller Uni- 
versity) and Dr. Nicholas Heintz (University of Ver- 
mont), Dr. Heintz and his colleagues have recently 
demonstrated that Oct-1 interacts with the dihydro- 
folate reductase {dhfr) origin of DNA replication 
and have identified novel activities (RIP60, 
RIP 100) that have properties expected of bona fide 
mammalian DNA replication-initiation factors. Thus 
RIP60 is a sequence-specific DNA-binding protein 
that interacts adjacent to an AT-rich, bent DNA seg- 
ment in the dhfr origin. It also binds specifically to 
two sites within the yeast ARS 1 (autonomously repli- 
cating sequence) origin of replication that have 
been shown to be essential for efficient ARSl func- 
tion. RIPlOO is an ~ 100-kDa ATP-binding protein 
that copurifies with significant DNA helicase activ- 
ity and interacts with RIP60. These and other fea- 
tures of the purified RIP60/RIP100 preparations 
suggest that they may be involved in chromosomal 
DNA synthesis. Further efforts are directed toward 
definitive demonstration that this is the case and in- 
vestigations into possible mechanisms for coordi- 
nate regulation of transcription and DNA replication 
during the S phase of the mammalian cell cycle. 
Dr. Heintz is also Professor of Molecular Biol- 
ogy at the Rockefeller University. 
Articles 
La Bella, F. , and Heintz, N. 1991. Histone gene tran- 
scription factor binding in extracts of normal hu- 
man cells. Mol Cell Biol 11:5825-5831. 
McKinney, J.D., and Heintz, N. 1991. Transcrip- 
tional regulation in the eukaryotic cell cycle. 
Trends Biochem Sci 16:430-435. 
Segil, N., Roberts, S.B., and Heintz, N. 1991. Cell- 
cycle-regulated phosphorylation of the transcrip- 
tion factor Oct- 1 . Cold Spring Harb Symp Quant 
Biol 56:285-292. 
Segil, N., Roberts, S.B., and Heintz, N. 1991. Mi- 
totic phosphorylation of the Oct- 1 homeodomain 
and regulation of Oct-1 DNA binding activity. 
Science 254:1814-1816. 
CELL BIOLOGY AND REGULATION 
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