FSY1, a novel gene encoding a specific fructose/H(+) symporter in the type strain of Saccharomyces carlsbergensis

Abstract

A novel gene, FSY1, encoding a permease involved in active fructose uptake by a proton symport mechanism in the type strain of Saccharomyces carlsbergensis has been isolated. Fsy1p is only distantly related to the Hxt proteins that mediate facilitated diffusion of glucose and fructose in Saccharomyces cerevisiae and related species.

FIG. 1

Illustration of the effect on extracellular pH of the addition of either 5 mM fructose (F) or 5 mM glucose (G) to aqueous fructose-grown cell suspensions (pH 5.0) of both the hxt-null strain (PYCC 5623) and the wild-type CEN.PK113-5D strain transformed with plasmid pT5 II. Increase in pH indicates proton uptake by the cells, and acidification indicates proton efflux, reflecting membrane ATPase activity.

FIG. 2

Eadie-Hofstee plot of the initial uptake rates of d-[¹⁴C]fructose measured in strain PYCC 5624, a S. cerevisiae hxt-null transformant carrying plasmid pT5 II. Cells were grown in YNB medium with 0.5% (wt/vol) fructose. Measurements for each concentration were made in triplicate. Kinetic parameters {V_max = 3.8 ± 0.2 mmol · h⁻¹ · g⁻¹ (dry weight [d.w.]); K_m = 0.16 ± 0.02 mM; 25°C, pH 5} were calculated using GraphPad Prism software, and the data were fitted to a one-component model according to Michaelis-Menten kinetics.

FIG. 3

Relationships between Fsy1p and other permeases belonging to the major facilitator superfamily. The dendrogram was obtained using Megalign software version 3.14 (Clustal method). All the permeases shown are S. cerevisiae proteins except the following: Fsy1, S. pastorianus; ltr1po and ltr2po, Schizosaccharomyces pombe; GalP, E. coli; Rco1, Neurospora crassa; and Rag1, Kluyveromyces lactis.