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is paired with IE1 and IE3 in bidirectional gene organization, a gene architecture
adopted from the mammalian host.
Stochastic Desilencing of the Major Immediate Early Locus
During Latency
In a model of latency established in the lungs, highly sensitive RT-PCRs detected
IE1 and IE2 transcripts in absence of any infectivity (Grzimek et al. 2001). For a
statistical analysis of the frequency of MIE locus transcription, the lungs were
subdivided into 18 pieces that were tested individually. This approach resulted in
patterns of transcript-positive and transcript-negative lung tissue pieces and allowed
calculation of transcription frequencies from the fraction of negative pieces by
using the Poisson distribution equation. Such a variegated expression, also referred
to as mosaic expression, is reminiscent of transgene expression (Fiering et al. 2000)
and indicates stochastic desilencing of an otherwise silenced MIE locus. It is
important to emphasize that MIE gene expression is not a constitutive, latency-
associated feature of latent mCMV genomes. MIE locus activity in latently infected
lungs was found to be approximately 10-20 events per 10
6
viral genomes. Therefore,
most viral genomes are in a state of MIE locus latency at any one time, with a minor
fraction moving a first step toward reactivation. We propose that all latent viral
genomes may be desilenced at the MIE locus some time. Unfortunately, this predic-
tion is difficult to test in a living organism.
Notably, desilencing of the bidirectional gene pair was found to be not coordi-
nated, indicating an independent regulation of the genes that flank the enhancer
region (Grzimek et al. 2001). This was true also after extrinsic signaling to the
enhancer, which showed that the enhancer does not synchronize transcription from
the bidirectional gene pair but rather operates as a switch (Simon et al. 2007). It is
currently open to question whether this feature is related to the tandem structure of
the mCMV enhancer region. According to our current understanding, isolated IE2
expression should not be able to initiate the replicative cycle. Interestingly, how-
ever, in the absence of extrinsic signaling to the enhancer, stochastic desilencing at
the
ie1
/
ie3
promoter did not initiate the replicative cycle either, as IE3 transcripts
were absent (Kurz et al. 1999). This identified IE3 splicing as a second molecular
checkpoint of latency and explained why replicative latency was maintained despite
MIE locus transcription.
Extrinsic Signals Triggering Transcriptional Reactivation
and Recurrence
Classical and more recent review articles have dealt in greater detail with extrinsic
signals and conditions, such as immune cell depletion, allogeneic transplantation,
ischemia/reperfusion injury, polymicrobial sepsis, inflammatory disease states in
