BetS is a major glycine betaine/proline betaine transporter required for early osmotic adjustment in Sinorhizobium meliloti.
Hybridization to a PCR product derived from conserved betaine choline carnitine transporter (BCCT) sequences led to the identification of a 3.4-kb Sinorhizobium meliloti DNA segment encoding a protein (BetS) that displays significant sequence identities to the choline transporter BetT of Escherichia coli (34%) and to the glycine betaine transporter OpuD of Bacillus subtilis (30%). Although the BetS protein shows a common structure with BCCT systems, it possesses an unusually long hydrophilic C-terminal extension (169 amino acids). After heterologous expression of betS in E. coli mutant strain MKH13, which lacks choline, glycine betaine, and proline transport systems, both glycine betaine and proline betaine uptake were restored, but only in cells grown at high osmolarity or subjected to a sudden osmotic upshock. Competition experiments demonstrated that choline, ectoine, carnitine, and proline were not effective competitors for BetS-mediated betaine transport. Kinetic analysis revealed that BetS has a high affinity for betaines, with K(m)s of 16 +/- 2 microM and 56 +/- 6 microM for glycine betaine and proline betaine, respectively, in cells grown in minimal medium with 0.3 M NaCl. BetS activity appears to be Na(+) driven. In an S. meliloti betS mutant, glycine betaine and proline betaine uptake was reduced by about 60%, suggesting that BetS represents a major component of the overall betaine uptake activities in response to salt stress. beta-Galactosidase activities of a betS-lacZ strain grown in various conditions showed that betS is constitutively expressed. Osmotic upshock experiments performed with wild-type and betS mutant cells, treated or not with chloramphenicol, indicated that BetS-mediated betaine uptake is the consequence of immediate activation of existing proteins by high osmolarity, most likely through posttranslational activation. Growth experiments underscored the crucial role of BetS as an emerging system involved in the rapid acquisition of betaines by S. meliloti subjected to osmotic upshock.
Boscari
Alexandre
A
Laboratoire de Biologie Végétale et Microbiologie, CNRS FRE 2294, Faculté des Sciences, Université de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cédex, France.
Mandon
Karine
K
Dupont
Laurence
L
Poggi
Marie-Christine
MC
Le Rudulier
Daniel
D
D001483:Base Sequence; D004926:Escherichia coli / Q000235:genetics; D026901:Membrane Transport Proteins / Q000235:genetics / Q000502:physiology*; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D011392:Proline / Q000031:analogs & derivatives* / Q000378:metabolism*; D016962:Sinorhizobium meliloti / Q000378:metabolism*; D014882:Water-Electrolyte Balance
D026901:Membrane Transport Proteins; C401955:glycine betaine transporter, Listeria monocytogenes; D011392:Proline; C003342:stachydrine