TRANS-REGULATION OF HIV-1 GENE EXPRESSION 
Bryan R. Cullen, Ph.D., Assistant Investigator 
Human immunodeficiency virus type 1 (HIV-l) is 
the predominant etiologic agent of acquired im- 
mune deficiency syndrome (AIDS). The disease 
AIDS may be viewed as the end stage of an ex- 
tended, chronic infection of the human host by the 
HIV-1 retrovirus. After the burst of viral replication 
that accompanies initial infection, HIV-l replication 
in the host becomes relatively quiescent. During 
this phase, which may be prolonged, low levels of 
viral replication appear to occur in cells that can act 
as a reservoir of HIV-1 infection, including various 
cells of the macrophage/monocyte lineage. This 
phase appears to be associated with a gradual de- 
cline in the CD4^ T cell count. Initial detectable 
disease eventually manifests itself as a less-severe 
immune deficiency, AIDS-related complex (ARC). 
Subsequent deterioration of the immune system 
then appears inexorable, leading finally to viremia 
and a dramatic depletion in the remaining CD4^ 
population. Clinical deterioration, marked by se- 
vere opportunistic infections and central nervous 
system degeneration, then ensues, finally leading to 
patient death. 
The replication of HIV-1 in the infected host may 
be seen to follow a complex, chronic course. This 
pattern is somewhat similar to that noted for the 
related animal retroviruses of the lentivirus type 
but is distinct from the acute replication typical of 
the majority of retroviruses, such as the avian leu- 
kemia virus (ALV) and murine leukemia virus 
(MLV). The chronicity of HIV-1 replication in the 
host may imply a complex balance between virus 
replication and host antiviral response. At least part 
of the ability of HIV-1 to achieve this balance may 
result from the complexity of its genetic makeup. 
Like other lentiviruses, but unlike ALV or MLy 
HIV-1 encodes nonstructural proteins, which act in 
trans to modulate viral gene expression. Functional 
expression of at least two of these. Tat and Rev, has 
been shown to be essential for viral replication in 
vitro. It is the aim of Dr. CuUen's laboratory to un- 
derstand how the expression of the genome of 
HIV-1 is regulated and, in particular, to delineate 
fully the roles and mechanisms of action of the 
HrV-1 Tat and Rev trans-activators. 
The HIV-1 Tat trans-activator acts to enhance 
greatly the expression of both viral and heterolo- 
gous sequences linked to the viral long terminal re- 
peat (LTR) promoter element. Dr. CuUen's labora- 
tory has presented evidence suggesting that Tat 
exerts this effect via a bimodal mechanism, re- 
sulting in both an increase in the rate of trans- 
cription of HIV-1 LTR-specific mRNAs and an en- 
hancement in the utilization of those mRNAs by 
the host cell translational machinery. The Tat pro- 
tein was, however, also shown to be localized 
predominantly to the nucleus of expressing cells, 
thus arguing against a direct role for Tat in influ- 
encing viral mRNA translation. The target sequence 
for Tat action, the trans-activation response (TAR) 
element, has been mapped to a transcribed re- 
gion of the HIV-1 LTR sequence immediately adja- 
cent to the site of transcription initiation. This re- 
gion has the potential to form a highly stable 
RNA stem-loop structure. Current evidence sug- 
gests that mutations that affect the integrity of this 
RNA structure also prevent Tat-mediated trans-acti- 
vation of HrV-1 gene expression. The possibility 
therefore exists that Tat may affect HIV-l gene ex- 
pression via an RNA-mediated, rather than DNA-me- 
diated, TAR sequence recognition event. This puta- 
tive protein-RNA interaction may be the key to at 
least an initial understanding of the mechanism of 
action of Tat. 
Although the tat gene product acts to enhance 
greatly the level of expression of any gene linked to 
HIV-1 LTR, the viral rev gene product acts post-tran- 
scriptionally to modulate specifically the expression 
of virally encoded mRNAs. In the absence of Rev, 
the incompletely spliced viral mRNAs that encode 
the virion structural proteins Gag and Env are syn- 
thesized at normal levels but excluded from the cell 
cytoplasm. They cannot, therefore, be functionally 
expressed. The HIV-1 Rev protein has been shown 
to induce the nuclear export of these viral mRNAs 
and therefore is essential for virion synthesis. A spe- 
cific target sequence for Rev function, the Rev re- 
sponse element (RRE), has been identified and 
shown to coincide with a highly stable, predicted 
RNA secondary structure contained within the 
HIV-1 env gene. Evidence suggests that this se- 
quence specificity is directly encoded within the 
Rev protein itself The HIV-l rev gene product is 
therefore the first eukaryotic regulatory protein 
that has been shown to regulate mRNA transport in 
a sequence-specific manner. 
A mutational analysis of the HIV-1 rev gene con- 
ducted in Dr CuUen's laboratory has provided ini- 
tial evidence for the existence of two functional 
domains within the HIV-1 Rev trans-activator equiv- 
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