In the accompanying paper, we demonstrated the presence of a fluorescence resonance energy transfer (FRET) between the tryptophans of the melibiose permease (MelB) of Escherichia coli and a fluorescent sugar, 2-(N-5-dimethylaminonaphthalene-1-sulfonyl)aminoethyl-1-thio-␤-D-galactopyranoside (Dns 2 -S-Gal) bound at the sugar-binding site (Maehrel, C., Cordat, E., Mus-Veteau, I., and Leblanc, G. (1998) J. Biol. Chem. 273, 33192-33197). To identify the tryptophans that transfer their energy to the

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... fluorescent sugar, we analyzed the FRET properties of MelB mutants carrying the replacement of each of the eight MelB tryptophans by a phenylalanine. The data indicate that Trp 64 , localized in loop 2-3 from the N-terminal domain, and Trp 299 , localized in helix IX in the C-terminal domain, are responsible for up to 80% of the FRET signal. Moreover, by assuming that only Trp 299 transfers energy to Dns 2 -S-Gal in mutant W64F, whereas only Trp 64 transfers energy to Dns 2 -S-Gal in mutant W299F, we calculated that Trp 299 and Trp 64 are about 14 and 20 Å away from the probe, respectively. In addition, we observed that mutating Trp 342 , localized in helix X of the C-terminal domain, produces a significant increase of the polarity of the fluorescent sugar environment, suggesting its proximity to the sugar-binding site. Taken together, these data provide additional support for the suggestion that (i) the sugarbinding site is localized in the C-terminal part of the transporter, probably close to membrane segments IX and X, and (ii) the N-terminal domain, and particularly cytoplasmic loop 2-3, is also close to the sugar-binding site.