A melibiose transporter and an operon containing its gene in Enterobacter cloacae

Abstract

We detected inducible melibiose transport activity in cells of Enterobacter cloacae IID977. H ؉ , but not Na ؉ , was found to be the coupling cation for this transporter. We cloned and sequenced the gene encoding the melibiose transporter. A homology search of a protein sequence database revealed that this melibiose transporter has high sequence similarity with the lactose transporter (LacY) and the raffinose transporter (RafB) and has some similarity with the melibiose transporter (MelB) of Escherichia coli.The melibiose transporter (MelB) and the lactose transporter (LacY) of Escherichia coli are secondary membrane transporters which mediate symport of galactosides and monovalent cations. These transporters have been revealed to be valuable systems for elucidation of structure-function relationships in cation-coupled symporters.

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

“…In LacY, these positions correspond to Met-83, Met-86, and Ala-177, respectively (7,22). According to the published hydropathy analysis of MelY (22,23) and the present topological models of LacY (9,16,34), Leu-88 and Leu-91 are predicted to reside in the middle of TMS-3 of MelY (Fig. 1).…”

“…The gene encoding the melibiose transporter, melY, has been cloned, sequenced (22), and found to reside in an inducible operon in the bacterial genome (23). The deduced amino acid sequence of the melY gene has shown a highly hydrophobic protein of 425 residues and 12 predicted transmembrane segments (TMS) (22).…”

“…The melibiose transporter of Enterobacter cloacae (MelY) is a secondary active transporter and catalyzes the symport (cotransport) of the ␣-galactoside sugar melibiose and cation (22,23). The gene encoding the melibiose transporter, melY, has been cloned, sequenced (22), and found to reside in an inducible operon in the bacterial genome (23).…”

“…Phylogenetic analyses show that the MelY transporter is a member of oligosaccharide H ϩ symporter family 5 and shares highly conserved amino acid sequence motifs with members of the major facilitator superfamily (MFS) (23,24,27,34). MelY is distinct from MelB (27) but shows high sequence identity (22,23) with the lactose carrier of Escherichia coli (LacY) (7), the raffinose transporter of E. coli (RafB) (1), the sucrose permease of E. coli (CscB) (2,26), the lactose carrier of Citrobacter freundii (LacY Cf ) (18), and the lactose carrier of Klebsiella pneumoniae (LacY Kp ) (20).…”

“…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.The melibiose transporter of Enterobacter cloacae (MelY) is a secondary active transporter and catalyzes the symport (cotransport) of the ␣-galactoside sugar melibiose and cation (22,23). The gene encoding the melibiose transporter, melY, has been cloned, sequenced (22), and found to reside in an inducible operon in the bacterial genome (23).…”