Biology Reference
In-Depth Information
IE86 protein because this region is necessary for dimerization of the IE86
protein in vivo. The various functions of the IE86 protein and their role in the
replication of HCMV are reviewed.
Autoregulation of the MIE Promoter
Negative autoregulation of the MIE promoter by the IE86 protein is almost cer-
tainly important to the replication of the virus because recombinant viruses that
fail to autoregulate can not be isolated (H. Isomura and
.F. Stinski, unpub-
lished data). The IE86 protein mutated at histidine residues 446 and 452 fails to
negatively autoregulate the MIE promoter in in vitro transcription reactions
(Macias and Stinski 1993), transient transfection assays (Macias et al. 1996),
and recombinant BACs (Petrik et al. 2007). The MIE promoter region of HCMV
has a
cis
-acting element that serves as a binding site for the IE86 protein or the
late L40 protein (see Fig. 1). Point mutations in histidine residues 446 and 452
abolish DNA binding (Macias and Stinski 1993; Macias et al. 1996). Dimerization
of the IE86 protein is required for viral DNA binding (Table 1) (Macias et al.
1996; Waheed et al. 1998). The viral DNA element is located immediately
upstream of the transcription start site for IE1/IE2 RNA and is referred to as the
cis
-repression sequence (crs). The crs consists of a pair of dinucleotide CG sepa-
rated by an A/T-rich region of 10 nucleotides. The CG dinucleotides are critical
and the spacing is important for binding by the IE86 protein (Waheed et al.
1998). The crs functions between the TATA box and the transcription start site
in either orientation. However, it does not function downstream of the transcrip-
tion initiation site (+1) (Pizzorno and Hayward 1990; Cherrington et al. 1991;
Liu et al. 1991). The IE86 protein binds to the minor groove between −14 and −1
without inhibiting the binding of TBP to the upstream TATA box (Jupp et al.
1993b; Lang and Stamminger 1994).
The first step in negative autoregulation of the MIE promoter by IE86 is block-
age of RNA polymerase II occupancy at the transcription start site (Wu et al. 1993;
Lee et al. 1996). There is also an initiator-like element between +1 and +7, and
cellular protein binding to this element is also affected by the IE86 protein, which
in turn negatively affects transcription from the MIE promoter (Macias et al.
1996). The ie3 gene product of murine CMV also negatively autoregulates its MIE
promoter (Messerle et al. 1992). Repression of the HCMV MIE promoter is
detectable approximately between 6 and 8 h after high multiplicity of infection
when the IE86 protein reaches levels to compete for binding to the transcription
initiation region (Stamminger et al. 1991; Meier and Stinski 1997).
The second stage of negative autoregulation by the IE86 protein, occurring 24 h
postinfection, requires histone deacetylase (HDAC1), which causes deacetylation
of histones at the MIE promoter (Reeves et al. 2006). Deacetylated histones are
targets for methylation by histone methyltransferases, G9a and Suvar(3-9)H1.
Since mutation of the crs abrogates the association of repressive chromatin to the
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