The five-protein 7-compIex particle is efficiently 
constituted from the five separate subunits. Func- 
tional studies show 7 and 5 are capable of assem- 
bling the 18 clamp, but they are inefficient in doing 
so and the 5' subunit is highly stimulatory, indicating 
an important role for 5' in assembling the (3 ring onto 
DNA. The 10-subunit polymerase holoenzyme is in 
the process of being assembled. These structural 
studies were supported by a grant from the National 
Institutes of Health. 
Epstein-Barr Virus Replication 
The Epstein-Barr virus (EBV) is the causative 
agent of infectious mononucleosis. After the initial 
infection, the EBV genome exists in a latent state as 
multiple copies of a 172-kb plasmid DNA in B lym- 
phocytes. Latent replication of EBV is tightly regu- 
lated, occurring only once per division of the host 
cell and in synchrony with the host chromosomes. 
Dr. Bill Sugden's laboratory has identified the viral 
latent origin of replication (oriP) and the single 
EBV-encoded gene product (EBNAl; EBV nuclear 
antigen 1) essential for latent EBV replication. Dr. 
O'Donnell's laboratory has overproduced the 
EBNAl protein in the baculovirus expression 
system. 
Study of EBNAl shows it induces a large DNA loop 
within the origin between the two essential ele- 
ments of oriP, the family of repeat (FR) element and 
the dyad symmetry (DS) element. The FR element is 
thought to be an enhancer of replication initiation at 
the DS element. The loop induced by EBNAl be- 
tween these two elements is reminiscent of loops 
generated by transcriptional enhancer proteins, im- 
plying similar methods are used to activate replica- 
tion. In another study, the EBNAl protein was found 
to induce a structural distortion in the DS element, 
perhaps an initial step to set the stage for entry of the 
host replication proteins into the origin. These stud- 
ies have been supported by the National Institutes 
of Health. Study of the role of EBNAl in regulated 
replication at oriP may further understanding of 
how eukaryotes regulate the replication of their 
chromosomes. 
Dr. O'Donnell is also Associate Professor of Mi- 
crobiology at Cornell University Medical College, 
New York City. 
Articles 
Bonner, C.A., Stukenberg, P.T., Rajagopalan, M., 
Eritja, R., O'DonneU, M., McEntee, K., Echols, 
H., and Goodman, M.F. 1992. Processive DNA syn- 
thesis by DNA polymerase II mediated by DNA 
polymerase III accessory proteins. / Biol Chem 
267:11431-11438. 
Frappier, L., and O'Donnell, M. 1991. Epstein-Barr 
nuclear antigen 1 mediates a DNA loop within the 
latent replication origin of Epstein-Barr virus. 
Proc Natl Acad Sci USA 88:10875-10879. 
Frappier, L., and O'Donnell, M. 1992. EBNAl dis- 
torts oriP, the Epstein-Barr virus latent replication 
origin. J Virol 66:1786-1790. 
Kong, X.-P., Onrust, R., O'Donnell, M., and 
Kuriyan, J. 1992. Three-dimensional structure 
of the |8 subunit of E. coli DNA polymerase III 
holoenzyme: a sliding DNA clamp. Cell 69:425- 
437. 
O'Donnell, M. 1992. Accessory protein function in 
the DNA polymerase III holoenzyme from E. coli. 
Bioessays 14:105-111. 
Onrust, R., Stukenberg, P.T., and O'Donnell, M. 
1991 . Analysis of the ATPase subassembly which 
initiates processive DNA synthesis by DNA poly- 
merase III holoenzyme./ 5/0/ Chem 266:21681- 
21686. 
Studwell-Vaughan, P.S., and O'Donnell, M. 1991. 
Constitution of the twin polymerase of DNA poly- 
merase III holoenzyme. /5«'o/ Chem 266:19833- 
19841. 
REGULATION OF THE ADRENAL STEROIDOGENIC ENZYMES 
Keith L. Parker, M.D., Ph.D., Assistant Investigator 
Dr. Parker's laboratory studies the mechanisms 
that regulate the adrenal steroidogenic enzymes. Ste- 
roid biosynthesis requires the concerted action of 
cytochrome P-450 enzymes that convert cholesterol 
to biologically active products. All steroidogenic 
cells express cholesterol side-chain cleavage en- 
zyme (SCC), whereas steroid 2 1 -hydroxylase (21- 
OHase) and 1 1 /3-hydroxylase (llj8-OHase) are 
limited to adrenocortical cells. Treatment of adreno- 
cortical cells with corticotropin (ACTH) coordi- 
nately increases the expression of all of these steroid 
hydroxylases; this induction is a major component 
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