Abstract
Threshold generation in fate-selection circuits is often achieved
through deterministic bistability, which requires cooperativity (i.e.,
nonlinear activation) and associated hysteresis. However, the Tat
positive-feedback loop that controls HIV's fate decision between
replication and proviral latency lacks self-cooperativity and
deterministic bistability. Absent cooperativity, it is unclear how HIV
can temporarily remain in an off-state long enough for the kinetically
slower epigenetic silencing mechanisms to act expression fluctuations
should rapidly trigger active positive feedback and replication,
precluding establishment of latency. Here, using flow cytometry and
single-cell imaging, we find that the Tat circuit exhibits a transient
activation threshold. This threshold largely disappears after similar to
40 h-accounting for the lack of deterministic bistability and promoter
activation shortens the lifetime of this transient threshold. Continuous
differential equation models do not recapitulate this phenomenon.
However, chemical reaction (master equation) models where the
transcriptional transactivator and promoter toggle between inactive and
active states can recapitulate the phenomenon because they intrinsically
create a single-molecule threshold transiently requiring excess
molecules in the inactive state to achieve at least one molecule (rather
than a continuous fractional value) in the active state. Given the
widespread nature of promoter toggling and transcription factor
modifications, transient thresholds may be a general feature of
inducible promoters.
