does not insert into target DNAs that already contain 
a copy of Tn7. Dr. Craig and her colleagues have 
recently established that Tn7 transposition immu- 
nity is active in the Escherichia coli chromosome. 
The work on the control of Tn7 was supported by 
a grant from the National Institutes of Health. 
Dr. Craig is also Associate Professor of Molecu- 
lar Biology and Genetics at the fohns Hopkins Uni- 
versity School of Medicine. 
Article 
Gamas, P., and Craig, N.L. 1992. Purification and 
characterization of TnsC, a Tn7 transposition pro- 
tein that binds ATP and DNA. Nucleic Acids Res 
20:2525-2532. 
MECHANISM OF RETROVIRUS INFECTION 
James M. Cunningham, M.D., Assistant Investigator 
Infection by retroviruses requires successful in- 
teraction between viral and host cell proteins. In the 
appropriate target cells, infection may result in 
transformation or cell death. Dr. Cunningham and 
his colleagues are interested in understanding how 
retroviruses infect cells and in determining the con- 
sequences of infection for the host. They have fo- 
cused on the behavior of a membrane protein that 
functions as a transporter of cationic amino acids 
and serves as a receptor for leukemogenic retrovi- 
ruses (murine leukemia virus, MuLV) in mice. 
Role of the Transporter in Infection 
Previous work in Dr. Cunningham's laboratory es- 
tablished that the transporter determines infection 
by binding to the MuLV envelope. Site-directed mu- 
tagenesis has identified a transporter domain that 
provides the site for virus attachment on the extra- 
cellular face of the plasma membrane. Binding to 
the transporter is a prerequisite for fusion of the 
virus envelope to the host cell membrane, which 
may occur after delivery of the virus to endosomes. 
Currently the laboratory is investigating the role of 
the transporter in these steps by screening for mu- 
tant transporter proteins that bind to the virus enve- 
lope but do not permit infection. The laboratory is 
also using crosslinking reagents and antibodies to 
look for host cell proteins that interact with the 
virus envelope and matrix proteins in the presence 
of the transporter. 
In the course of the experiments designed to 
identify mutant transporter proteins, Dr. Cun- 
ningham and his co-workers recognized that synthe- 
sis of complementary DNA from the viral RNA tem- 
plate begins within virions prior to infection. 
Previously it had been believed that replication of 
the MuLV genome by reverse transcriptase was initi- 
ated in the cytoplasm of the host cell after infection. 
These findings demonstrate the importance of the 
cleavage of active reverse transcriptase from the 
virus-encoded gag-pol polyprotein within maturing 
virions. In addition, they suggest that inhibition of 
retrovirus replication need not occur within in- 
fected cells. 
Consequences of Retrovirus Infection 
for the Host Cell 
Dr. Cunningham and his colleagues have been in- 
vestigating virus interference — the resistance of in- 
fected cells to additional infection by MuLVs that 
use the same receptor. Interference is a conse- 
quence of binding of newly synthesized virus enve- 
lope to the transporter. Binding of the MuLV enve- 
lope protein to the transporter in the endoplasmic 
reticulum results in a decrease in the delivery of the 
transporter to the cell surface, increased turnover of 
the transporter, and an increase in the steady-state 
level of the transporter-encoding mRNA. Currently 
the laboratory is testing the hypothesis that the in- 
crease in transporter mRNA within infected cells 
may be a response to depletion of intracellular cat- 
ionic amino acids, which results from the decrease 
in functional transporter at the plasma membrane. 
In addition, altered glycosylation of the transporter 
in infected cells has been observed, and two aspara- 
gine residues within the envelope-binding domain 
have been identified that are the sites for glycosyla- 
tion. The modification of N-linked carbohydrate 
during passage of the transporter through the Golgi 
may be altered when bound to envelope protein. 
Subsequent experiments using site-directed muta- 
genesis have excluded a requirement for glycosyla- 
tion in transporter-mediated amino acid uptake and 
MuLV infection. The possibility that an altered car- 
bohydrate structure on the transporter may be im- 
portant for interference is now being examined. 
CELL BIOLOGY AND REGULATION 4 1 
