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Journal of Bacteriology, January 2008, p. 168-178, Vol. 190, No. 1
0021-9193/08/$08.00+0     doi:10.1128/JB.01509-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Two Closely Related ABC Transporters in Streptococcus mutans Are Involved in Disaccharide and/or Oligosaccharide Uptake

Alexander J. Webb, Karen A. Homer, and Arthur H. F. Hosie^*

King's College London, Microbiology, Dental Institute, London, United Kingdom

Received 19 September 2007/ Accepted 12 October 2007

Streptococcus mutans has a large number of transporters apparently involved in the uptake of carbohydrates. At least two of these, the multiple sugar metabolism transporter, MsmEFGK, and the previously uncharacterized MalXFGK, are members of the ATP-binding cassette (ABC) superfamily. Mutation analysis revealed that the MsmEFGK and MalXFGK transporters are principally involved in the uptake of distinct disaccharides and/or oligosaccharides. Furthermore, the data also indicated an unusual protein interaction between the components of these two related transporters. Strains lacking msmE (which encodes a solute binding protein) can no longer utilize raffinose or stachyose but grow normally on maltodextrins in the absence of MalT, a previously characterized EII^mal phosphotransferase system component. In contrast, a mutant of malX (which encodes a solute binding protein) cannot utilize maltodextrins but grows normally on raffinose or stachyose. Radioactive uptake assays confirmed that MalX, but not MsmE, is required for uptake of [U-¹⁴C]maltotriose and that MalXFGK is principally involved in the uptake of maltodextrins with as many as 7 glucose units. Surprisingly, inactivation of the corresponding ATPase components did not result in an equivalent abolition of growth: the malK mutant can grow on maltotetraose as a sole carbon source, and the msmK mutant can utilize raffinose. We propose that the ATPase domains of these ABC transporters can interact with either their own or the alternative transporter complex. Such unexpected interaction of ATPase subunits with distinct membrane components to form complete multiple ABC transporters may be widespread in bacteria.

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* Corresponding author. Mailing address: Microbiology, King's College London Dental Institute, Floor 28, Guy's Tower, King's College London, Guy's Campus, London, SE1 9RT, United Kingdom. Phone: 44 (0) 20 7188 1825. Fax: 44 (0) 20 7188 3871. E-mail: arthur.hosie{at}kcl.ac.uk

Published ahead of print on 26 October 2007.

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Journal of Bacteriology, January 2008, p. 168-178, Vol. 190, No. 1
0021-9193/08/$08.00+0     doi:10.1128/JB.01509-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

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