Studies on the Coiled-Coil Protein TlpA in
Salmonella: A Thermosensing Transcription Autoregulator
The coiled-coil motif is composed of amino acid heptad repeats with
hydrophobic or apolar residues at conserved positions. Such repeats are
responsible for
the apolar stripe that is buried inside the coiled-coil structure formed
upon coiling of
two (or more) alpha-helical chains about each other.
The Salmonella typhimurium virulence plasmid carries the tlpA gene which
encodes a protein with an extensive coiled-coil domain. TlpA was shown to
be a
sequence-specific DNA-binding protein. The DNA-binding domain is situated
at the N-
terminus and attached to it is the elongated coiled-coil domain extending
all the way to
the C-terminus. TlpA acts at the tlpA promoter as a transcription
repressor.
TlpA structure is governed by a monomer to coiled-coil equilibrium.
Therefore
the fully folded coiled-coil in TlpA is inherently concentration and
temperature
dependent. Shifting exponentially growing Salmonella to temperatures
ranging from 37
°C to 45 °C leads to increasing derepression of the tlpA promoter.
Interestingly this is
the temperature range met by an invading bacterial pathogen in the host
organism. It
was demonstrated that the regulation of tlpA transcription is exerted by
the TlpA
repressor protein only. In vitro DNA-binding tested with highly purified
TlpA was also
temperature dependent. Control experiments ruled out the involvement of
supercoiling
and temperature induced changes in DNA topology and dependence on heat
shock
response. The thermoregulatory capacity of TlpA is due to its monomer to
coiled-coil
equilibrium which is pushed more to the monomer side (monomers being
unfolded and
nonfunctional) at elevated temperatures leading to subsequent
derepression of tlpA and
increased transcription.
Studies on TlpA are unique in that they demonstrate for the first time
that a
single protein can regulate transcription according to sensed temperature
cues. TlpA
also represents a novel utilization of the dynamic coiled-coil structure
for regulation of
function.
Key words: coiled-coil/gene
regulation/repressor/thermoregulation/Salmonella
ISBN 9 1-628-2304-
