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Analysis of the Major Hexose Transporter Genes in Wine Strains of Saccharomyces cerevisiae

¹ Graduate student, and ² Professor and Maynard A. Amerine Endowed Chair, Department of Viticulture and Enology, University of California, Davis, CA, 95616; ³ current address, Visiting professor, Joint Science Department, The Claremont Colleges, W.M. Keck Science Center, 925 N. Mills Ave., Claremont, CA 91711.

^* Corresponding author (email: lfbisson{at}ucdavis.edu; tel: 530 752-3835; fax: 530 752-0382)

Saccharomyces cerevisiae maintains a large family of hexose transporters encoded by the HXT genes. The major transporter genes, HXT1 through HXT7, were sequenced from four vineyard isolates and two commercial wine yeast strains and compared to the sequences in the Saccharomyces Genome Database for strain S288C and to those available for two additional wine strains V5 and RM11-1a. Base pair changes leading to differences in amino acid sequence were found for all seven transporters. Differences ranged from none to eight amino acid variations for the sequenced strains, depending upon the strain and the gene, in comparison with S288C. In contrast, RM11-1a displayed high degeneracy with multiple in-frame stop mutations for HXT1, HXT4, and HXT6. Several wine strain sequences for the HXT4 gene contained an identical additional 16 amino acids at the C-terminus. Transporter protein levels were analyzed in a wine yeast strain (UCD932) using green fluorescent protein tagging. HXT5, not shown to be expressed in previous studies, was expressed in UCD932 during fermentation. Expression of HXT4, a prominently expressed transporter in laboratory media, was not detected. Deletion of HXT1, HXT3, or HXT5 did not result in a discernable phenotype in UCD932 under the fermentation conditions used in this study as compared with the wild type strain. However, the strain lacking HXT3 was unable to complete the fermentation in media containing 5% exogenous ethanol. This result suggests that correct expression of HXT3 may play a role in ethanol tolerance.

Key words: yeast, sugar transport, fermentation, HXT gene