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Signal
transduction networks and the control of gene expression
Networks of signaling pathways
permit cells to sense and respond to their changing
environment. Some of these signaling pathways produce
small molecules, called second messengers, that mediate
a wide range of cellular responses. These include cyclic
AMP (cAMP) and cyclic di-GMP (c-di-GMP). Other signaling
pathways do not produce small molecules but, instead,
respond to them. For example, certain members of the
family of two-component signal transduction (2CST)
pathways respond to the small molecule acetyl phosphate
(acetyl-P). In the Wolfe lab, we study the impact of all
three of these small molecules.
The well-studied cAMP binds to a
transcription factor called CRP (also known as CAP),
permitting it to bind to DNA and activate the
transcription of hundreds of genes. One of those genes
is acs, which encodes acetyl-CoA synthetase (Acs),
the enzyme that converts free acetate into acetyl-CoA.
In humans, Acs re-activates the acetate stripped from
acetylated histones, removed from acetylcholine, or
derived by the metabolism of the ethanol I drank last
night. In bacteria, like Escherichia coli, it
performs a critical survival function, scavenging for
small amounts of acetate in the environment during
periods of carbon starvation. The critical role of Acs
requires exquisite regulation, especially at the level
of transcription initiation. In an effort to understand
how cells control the function of RNA polymerase, the
bio-machine responsible for transcription, we have been
dissecting the sophisticated circuitry that regulates acs
transcription.
The newly recognized c-di-GMP has
been implicated in the transition between the motile/planktonic
and sessile/biofilm lifestyles of diverse bacteria,
including diverse pathogens. Many bacteria possess
dozens of genes implicated in the synthesis and
degradation of c-di-GMP, suggesting that they produce c-di-GMP
for use in multiple pathways, presumably in response to
unique stimuli. In most cases, however, the identity of
c-di-GMP targets and the mechanisms by which this
molecule identifies and acts upon those targets remain
unknown. Also poorly understood are the mechanisms by
which the levels and/or localization of c-di-GMP are
regulated. In close collaboration with my colleague and
neighbor Dr. Karen Visick, we have gained valuable
insights into the control of c-di-GMP production and its
targets while investigating the motility of the marine
bacterium, Vibrio fischeri.
Acetyl-P (derived directly from
acetyl-CoA) has been reported to act as a global signal
with the capacity to donate phosphoryl groups directly
to a subset of 2CST regulators (RRs). Previously, we
employed DNA array technology to demonstrate that
acetyl-P influences the transcription of nearly 100
genes. More recently, through a series of epistasis
experiments, we showed that acetyl-P acts through RcsB,
a global RR effecting transitions between the motile and
sessile lifestyles. Currently, we are completing studies
that support the hypothesis that acetyl-P affects
transcription by directly donating its phosphoryl group
to RRs such as RcsB. Taken together, these observations
establish a connection between central metabolism and
cell signaling mediated by acetyl-P. Future studies
include mapping the impact of acetyl-P on the entire
network of 2CST pathways, learning how cells regulate
the levels of acetyl-P, and developing new assays to
monitor acetyl-P, especially in pathogens with stringent
growth conditions.
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