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J. Bacteriol., Nov 1997, 6979-6985, Vol 179, No. 22
A Verheul, E Glaasker, B Poolman and T Abee
The naturally occurring compatible solutes betaine and L-carnitine allow
the food-borne pathogen Listeria monocytogenes to adjust to environments of
high osmotic strength. Previously, it was demonstrated that L.
monocytogenes possesses an ATP-dependent L-carnitine transporter (A.
Verheul, F. M. Rombouts, R. R. Beumer, and T. Abee, J. Bacteriol.
177:3205-3212, 1995). The present study reveals that betaine and
L-carnitine are taken up by separate highly specific transport systems and
support a secondary transport mechanism for betaine uptake in L.
monocytogenes. The initial uptake rates of betaine and L- carnitine are not
influenced by an osmotic upshock, but the duration of transport of both
osmolytes is directly related to the osmotic strength of the medium.
Regulation of uptake of both betaine and L-carnitine is subject to
inhibition by preaccumulated solute. Internal betaine inhibits not only
transport of external betaine but also that of L- carnitine and, similarly,
internal L-carnitine inhibits transport of both betaine and L-carnitine.
The inhibition is alleviated upon osmotic upshock, which suggests that
alterations in membrane structure are transmitted to the allosteric binding
sites for betaine and L-carnitine of both transporters at the inner surface
of the membrane. Upon osmotic downshock, betaine and L-carnitine are
rapidly released by L. monocytogenes as a consequence of activation of a
channel-like activity. The osmolyte-sensing mechanism described is new and
is consistent with various unexplained observations of osmoregulation in
other bacteria.
Copyright © 1997, American Society for Microbiology
Betaine and L-carnitine transport by Listeria monocytogenes Scott A in response to osmotic signals
Department of Food Science, Wageningen Agricultural University, The Netherlands.
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