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J. Bacteriol., 11 1996, 6216-6222, Vol 178, No. 21
A Boorsma, ME van der Rest, JS Lolkema and WN Konings
Citrate uptake in Bacillus subtilis is mediated by a secondary transporter
that transports the complex of citrate and divalent metal ions. The gene
coding for the transporter termed CitM was cloned, sequenced, and
functionally expressed in Escherichia coli. Translation of the base
sequence to the primary sequence revealed a transporter that is not
homologous to any known secondary transporter. However, CitM shares 60%
sequence identity with the gene product of open reading frame N15CR that is
on the genome of B. subtilis and for which no function is known. The
hydropathy profiles of the primary sequences of CitM and the unknown gene
product are very similar, and secondary structure prediction algorithms
predict 12 transmembrane-spanning segments for both proteins. Open reading
frame N15CR was cloned and expressed in E. coli and was shown to be a
citrate transporter as well. The transporter is termed CitH. A remarkable
difference between the two transporters is that citrate uptake by CitM is
stimulated by the presence of Mg2+ ions, while citrate uptake by CitH is
inhibited by Mg2+. It is concluded that the substrate of CitM is the
Mg(2+)-citrate complex and that CitH transports the free citrate anion.
Uptake experiments in right-side-out membrane vesicles derived from E. coli
cells expressing either CitM or CitH showed that both transporters catalyze
electrogenic proton/substrate symport.
Copyright © 1996, American Society for Microbiology
Secondary transporters for citrate and the Mg(2+)-citrate complex in Bacillus subtilis are homologous proteins
Department of Microbiology, Groningen Biotechnology and Biomolecular Sciences Institute, University of Groningen, The Netherlands.
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