greatly delayed kinetics. This virus also infects 
woodchucks, allowing determination of whether 
host or viral factors control these biological differ- 
ences. Studies of GSHV-infected woodchucks dem- 
onstrated that GSHV was potently oncogenic, but 
again tumors appeared only late in life. Moreover, 
molecular analysis of tumor DNA revealed that 
N-myc loci are not rearranged in these tumors. At- 
tempts to define potential targets of GSHV insertion 
in these tumors are now under way. 
Viral Replication 
Work also continues on the molecular basis of he- 
padnaviral replication. This process is of great in- 
trinsic interest because the replication of these DNA 
viruses proceeds through reverse transcription of an 
RNA intermediate. This reaction is formally analo- 
gous to that involved in retroviral replication but 
differs mechanistically in many major ways. A fuller 
understanding of this process should inform studies 
of other reverse transcription reactions and may also 
define novel potential targets for antiviral therapy. 
Replicative intermediates in hepadnaviral replica- 
tion have been identified through the study of viral 
mutants blocked at various stages of minus- and 
plus-strand DNA synthesis. These studies have led 
Dr. Ganem and his colleagues to the first mechanis- 
tic understanding of how duplex linear viral DNA is 
generated and how the RNA primer for plus-strand 
synthesis is cleaved from genomic RNA. The latter is 
accomplished not by recognition of specific se- 
quences, as had previously been thought, but by 
measurement of the distance from the 5' end of the 
transcript. 
The genomic RNA of hepadnaviruses is terminally 
redundant, and each copy of the redundancy har- 
bors potential polyadenylation signals. Thus synthe- 
sis of full-length RNA requires that the 5' poly(A) 
signals be bypassed, while the 3' signals are effi- 
ciently utilized. Experiments addressing this inter- 
esting form of regulation revealed that positioning 
of the poly(A) signals close to a cap site greatly sup- 
presses their recognition; the mechanism of this 
suppression is unknown. Other, more complex, ele- 
ments also contribute (to a lesser degree) to this 
regulation. 
Once replication is complete, progeny viral nu- 
cleocapsids bud into the endoplasmic reticulum, 
thereby acquiring their complement of the three en- 
velope glycoproteins L, M, and S. Mutant viruses 
bearing lesions in each coding region were con- 
structed and examined, to explore the functions of 
these proteins in viral assembly. Only the L and S 
proteins were required for assembly and secretion; 
the function of the M protein remains a mystery. 
Another role of the envelope proteins is to bind 
the cellular receptor for the virus, thereby initiating 
viral entry. In recent work, cellular polypeptides 
that bind viral envelope proteins have been sought 
by biochemical assays. In this way a host glycopro- 
tein of 180 kDa has been identified that binds the L 
protein of the avian hepatitis B virus with high affin- 
ity. Binding can be blocked by neutralizing mono- 
clonal antibodies. Clones for this protein have been 
obtained and their characterization is under way. 
Work on hepadnaviral replication is supported by 
the National Institutes of Health. 
Dr. Ganem is also Professor of Microbiology 
and Immunology and of Medicine (Infectious 
Diseases) at the University of California, San 
Francisco. 
Articles 
Bruss, v., and Ganem, D. 1991 • Mutational analysis 
of hepatitis B surface antigen particle assembly 
and secretion . / Virol 65:3813-3820. 
Bruss, v., and Ganem, D. 1991. The role of enve- 
lope proteins in hepatitis B virus assembly. Proc 
Natl Acad Sci USA 88:1059-1063. 
Cherrington, J., and Ganem, D. 1992. Regulation of 
polyadenylation in human immunodeficiency 
virus (HIV) : contributions of promoter proximity 
and upstream sequences. EMBO f 11:1513- 
1524. 
Hirsch, R.C., Loeb, D.D., Pollack, J. R., and Ganem, 
D. 1991. c/5-Acting sequences required for en- 
capsidation of duck hepatitis B virus pregenomic 
RNA. / Wro/ 65:3309-3316. 
Loeb, D., Hirsch, R.C., and Ganem, D. 1991. Se- 
quence-independent RNA cleavages generate the 
primers for plus strand DNA synthesis in hepatitis 
B viruses: implications for other reverse transcrib- 
ing elements. EMBO f 10:3533-3540. 
Perara, E., Ganem, D., and Engel, J.N. 1992. A de- 
velopmentally regulated chlamydial gene with ap- 
parent homology to eukaryotic histone H 1 . Proc 
Natl Acad Sci USA 89:2l25-2\29. 
Seeger, C, Baldwin, B., Hornbuckle, W.E., Yeager, 
A.E., Tennant, B.C., Cote, P., Ferrell, L., Ganem, 
D., and Varmus, H.E. 1991. Woodchuck hepatitis 
virus is a more efficient oncogenic agent than 
ground squirrel hepatitis virus in a common host. 
/ Wro/ 65:1673-1679. 
Staprans, S., Loeb, D.D., and Ganem, D. 1991. Mu- 
tations affecting hepadnavirus plus-strand DNA 
synthesis dissociate primer cleavage from translo- 
cation and reveal the origin of linear viral DNA. / 
K/ro/ 65:1255-1262. 
CELL BIOLOGY AND REGULATION 57 
