Baculovirus Replication
Gene expression in baculoviruses
can be divided into a distinct phases based on when in the baculovirus replication
cycle the
gene
is expressed. These phases are the early, late and very late (or occlusion).
Some genes can be expressed in more than one phase of the replication cycle.
Control of the expression is mainly by the promoter region of the gene. Genes
which have promoter elements with strong similarity to insect promoters tend
to be expressed early in the infection, whereas genes with specific viral
promoter sequences tend to be expressed during the later phases of the replication
cycle.
Immediate Early
and Delayed Early
During the early phase of infection genes involved
in the regulation of the replication cascade (IE-0, IE-1, IE-2, PE-38) and
those involved in preventing host responses are expressed (eg p35). The genes
expressed in the early phase of infection and replication can be divided into
immediate early and delayed early. The immediate early genes do not require
viral transregulators for efficient expression and include the transregulators
(IE-1, IE-2). The delayed early genes are characterised by the requirement
of a transregulator for efficient transcription.
Genes required for DNA synthesis [DNA polymerase (dnapol)
and a DNA helicase-like protein (p143)] and factors involved in late gene
expression (eg lefs - late expression factors) are also expressed in
the early phase. A number of genes which modify aspects of the intra- and
extracellular environment are also expressed at this stage. The enzyme EGT
is produced to modify the extracellular environment (see below) and p35 (as
well as various IAPs) is produce to prevent apoptosis (see below).
The early genes of the baculovirus are transcribed
by a host RNA polymerase, however many of the early promoters also require
the regulatory protein IE-1 (or IE-0) for efficient transcription. Early genes
have variable transcription patterns. The transcripts of some are present
as early as 1 hour post-infection (h.p.i.) and decline quickly (by 6 h.p.i.),
whereas others accumulate with time. This difference in the presence of transcripts
may reflect the roles of the various early proteins and the presence of a
late promoter motif. The timing of the transcripts varies between viruses
and also depends on the species of host that the virus is infecting.
Late Phase
The late phase of baculoviral replication begins at
approximately 6 h.p.i. and extends through to to about 24 h.p.i. The features
of this phase include the replication of the viral DNA, the shutdown of host
cell transcription and translation and the production of the budded form of
the virus. Proteins produced in this phase include the DNA binding protein
p6.9 (involved in DNA packaging) and GP64 (envelope fusion protien found on
the surface of the budded virus). The switch from early to late gene expression
involves a change in the RNA polymerase used for the transcription of genes.
The polymerase utilised in this phase is a virally encoded RNA polymerse.
This form of polymerase recognises a different promoter sequence (TAAG) to
the sequences recognised in the early phase of replication (TATA-like and/or
CAGT motifs).
One of the key features of the late phase is the replication
of the genome. This replication is thought to originate at the hr sequences
and proceed around the genome. This method of DNA replication is referred
to a rolling circle method. As there are multiple hrs in the baculoviral genome
this would allow DNApol to bind at multiple sites and thus replicate the genome
in the most efficient way.
Nucloecapsids are also formed during the late phase.
This involves the production of the helical nucleocapsids and the packaging
of the viral DNA. Packaging fo the viral DNA requires the dephosphorylation
of p6.9. The fate of the nucleocapsids is variable. They can either bud out
through the cellular membrane and disseminate the infection within the insect
by infecting other cells by GP64 mediated envelope fusion and subsequent endocytosis,
or be occluded for horizontal transmission.
Very Late
(or Occlusion) Phase
The very late phase of of replication is also called
the occlusion phase. In this stage the virus becomes occluded in the protein
polyhedrin and the polyhedral envelope (calyx) is produced. The final stages
of this phase involves the lysis of the cell and release of the occluded virus.
This lysis results in the death and liquefaction of the host. The viral proteins
cathepsin and chitinase are involved in the liquefaction process. Cathepsin
breaks down the membranes and cell walls within the insect while the chitinase
degrades the chitin in the exoskeleton of the larvae.
An general overveiw of the replication cycle of baculoviruses
is shown below.
© Kalmakoff & Ward
University of Otago, Dunedin, New Zealand, 2003.