Biology Reference
In-Depth Information
γ
34.5
ICP0
a
RR
1/2
ICP22
I
CP
4
IC
P
27
LA
T
UL
US
a b
b' a' c'
ca
LAT
ICP0
γ
34.5
ICP4
TK
vhs VP16
ICP47
Infection
Latency
b
Lytic cycle
VP16, vhs…
packaged in the
virion
Virions
Viral DNA
replication
IE
or
a
ICP0
ICP4
ICP22
ICP27
ICP47
E
or ß
DNA
replicating
proteins
TK, RR1/2,
UL30/42…
L
or
g
Structural
proteins:
VP16, vhs,
γ
34.5
,
Us11…
Fig. 3
Virus gene expression and the lytic viral cycle. (
a
) Position of the IE genes referred in the main text, as well
as of some E and L genes, on the viral genome. (
b
) During productive (lytic) infection, the viral genome expres-
sion program occurs following three sequential steps that are coordinately regulated, leading to the synthesis
of immediate-early (IE or
α
), early (E or
β
), and late (L or
γ
) proteins
(Fig.
3a
). Additional genes and open reading frames that overlap
with previously recognized canonical genes have been more
recently identifi ed, and the total number of genes encoding pro-
teins amounts today up to 80. In addition, it has very recently been
discovered that the viral genome expresses at least 6 microRNA
(miRNA), most of which are encoded in the repeated
b
sequences
[
4
]. Lastly, the viral genome contains two types of cis-acting
sequences that are essential for virus replication: 3 origins of DNA
synthesis, one located in UL (oriL) and the other two in the repeated
c
sequences surrounding US (oriS), and the cleavage/packaging
sequences (sometimes referred as
pac
), which are contained in the
repeated
a
sequences. Mutant viruses lacking either oriL or both
copies of oriS are replication competent, suggesting that all origins
are functionally equivalent. It should be noted that a single origin of
DNA replication (either oriL or oriS) and a single
a
sequence are
enough to warrant replication and packaging of the virus genome.
5
Virus Gene Expression
During lytic infection, expression of the 80 HSV-1 genes is strictly
temporarily regulated in a cascade fashion, giving rise to three
phases of gene expression (Fig.
3b
). This expression cascade, which
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