Fig. 2. Possible mechanisms by which ubiquitin-mediated proteolysis may regulate transcription of hormoneinducible
genes. Model (a): In the absence of a signal, constitutively
expressed transcription activators
are prevented
from
activating
hormone-inducible genes because of their heterodimerization with repressors.
Upon hormone
exposure,
the repressors
are targeted for proteasome-mediated degradation and the released transcription
activators
switch
on hormone-inducible genes, including those encoding the repressor proteins. The
de
novo
synthesis of transcription repressors leads to the re-formation of heterodimers that prevent transcription of
hormone-inducible genes. The system is now primed for a fresh round of response to a hormone signal. This
strategy is used in the auxin signaling mediated by AUX/IAA-auxin response factor (ARF) proteins. Model (b): In the
absence of a signal, transcription of downstream hormone-inducible genes is off as transcription activators are in an
inactive state or their genes are not expressed. Upon hormone exposure, specific transcription factors are activated
or their genes induced. The resulting active transcription factors bind to hormone-inducible promoters to turn on
their transcription. These transcriptional activators have short half-lives as they are rapidly ubiquitylated and
degraded via the 26S proteasomes. Therefore, hormone-inducible genes are expressed as long as the signal is
present but transcription is switched off immediately upon the signal termination. Model (c): Similar to model (b);
specific transcription factors are induced upon hormone exposure to regulate expression of downstream hormoneinducible
genes. These transcription factors
are normally very unstable because of ubiquitin-mediated proteolysis.
In
the presence of the hormone, they are somehow stabilized
through an unknown mechanism
that might involve
altered
ubiquitylation or downregulation of specific E3 enzymes. This ensures an amplification of the response in
the
presence of a signal as both transcription is induced and protein turnover
is reduced. In contrast, as soon as the
signal
is terminated, these factors
become extremely unstable and are rapidly degraded through a proteasomedependent
pathway,
priming the system for response to a new stimulus. This mechanism
probably occurs
in the
abscisic
acid (ABA) signaling mediated by the bZIP factor ABI5.