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J. Bacteriol., Sep 1996, 5071-5079, Vol 178, No. 17
RM Kappes, B Kempf and E Bremer
The accumulation of the osmoprotectant glycine betaine from exogenous
sources provides a high degree of osmotic tolerance to Bacillus subtilis.
We have identified, through functional complementation of an Escherichia
coli mutant defective in glycine betaine uptake, a new glycine betaine
transport system from B. subtilis. The DNA sequence of a 2,310-bp segment
of the cloned region revealed a single gene (opuD) whose product (OpuD) was
essential for glycine betaine uptake and osmoprotection in E. coli. The
opuD gene encodes a hydrophobic 56.13- kDa protein (512 amino acid
residues). OpuD shows a significant degree of sequence identity to the
choline transporter BetT and the carnitine transporter CaiT from E. coli
and a BetT-like protein from Haemophilus influenzae. These membrane
proteins form a family of transporters involved in the uptake of
trimethylammonium compounds. The OpuD- mediated glycine betaine transport
activity in B. subtilis is controlled by the environmental osmolarity. High
osmolarity stimulates de novo synthesis of OpuD and activates preexisting
OpuD proteins to achieve maximal glycine betaine uptake activity. An opuD
mutant was constructed by marker replacement, and the OpuD-mediated glycine
betaine uptake activity was compared with that of the previously identified
multicomponent OpuA and OpuC (ProU) glycine betaine uptake systems. In
addition, a set of mutants was constructed, each of which synthesized only
one of the three glycine betaine uptake systems. These mutants were used to
determine the kinetic parameters for glycine betaine transport through
OpuA, OpuC, and OpuD. Each of these uptake systems shows high substrate
affinity, with Km values in the low micromolar range, which should allow B.
subtilis to efficiently acquire the osmoprotectant from the environment.
The systems differed in their contribution to the overall glycine betaine
accumulation and osmoprotection. A triple opuA, opuC, and opuD mutant
strain was isolated, and it showed no glycine betaine uptake activity,
demonstrating that three transport systems for this osmoprotectant operate
in B. subtilis.
Copyright © 1996, American Society for Microbiology
Three transport systems for the osmoprotectant glycine betaine operate in Bacillus subtilis: characterization of OpuD
Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
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