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J. Bacteriol. doi:10.1128/JB.01509-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Two closely related ABC transporters in Streptococcus mutans are involved in di/oligosaccharide uptake

King's College London, Microbiology, Dental Institute, London, UK

^* To whom correspondence should be addressed. Email: arthur.hosie{at}kcl.ac.uk.

	Abstract

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 di/oligosaccharides. Furthermore, 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 PTS system. In contrast, a malX (which encodes a solute binding protein) mutant 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 up to seven 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 its 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.