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Journal of Bacteriology, June 2003, p. 3672-3677, Vol. 185, No. 12
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.12.3672-3677.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Altered Substrate Selection of the Melibiose Transporter (MelY) of Enterobacter cloacae Involving Point Mutations in Leu-88, Leu-91, and Ala-182 That Confer Enhanced Maltose Transport

Steven G. Shinnick, Stephanie A. Perez, and Manuel F. Varela*

Department of Biology, Eastern New Mexico University, Portales, New Mexico 88130

Received 14 November 2002/ Accepted 1 April 2003

We isolated mutants of Escherichia coli HS4006 containing the melibiose-H+ symporter (MelY) from Enterobacter cloacae that had enhanced fermentation on 1% maltose MacConkey plates. DNA sequencing revealed three site classes of mutations: L-88-P, L-91-P, and A-182-P. The mutants L-88-P and L-91-P had 3.6- and 5.1-fold greater maltose uptake than the wild type and enhanced apparent affinities for maltose. Energy-coupled transport was defective for melibiose accumulation, but detectable maltose accumulation for the mutants indicated that active transport is dependent upon the substrate transported through the carrier. We conclude that the residues Leu-88, Leu-91 (transmembrane segment 3 [TMS-3]), and Ala-182 (TMS-6) of MelY mediate sugar selection. These data represent the first MelY mutations that confer changes in sugar selection.

* Corresponding author. Mailing address: Department of Biology, Eastern New Mexico University, Station 33, Portales, NM 88130. Phone: (505) 562-2464. Fax: (505) 562-2192. E-mail: Manuel.Varela{at}enmu.edu.

Journal of Bacteriology, June 2003, p. 3672-3677, Vol. 185, No. 12
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.12.3672-3677.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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