Truncation of Gal4p explains the inactivation of the GAL/MEL regulon in both Saccharomyces bayanus and some Saccharomyces cerevisiae wine strains

Abstract

In the past, the galactose-negative (Gal-) phenotype was a key physiological character used to distinguish Saccharomyces bayanus from S. cerevisiae. In this work, we investigated the inactivation of GAL gene networks in S. bayanus, which is an S. uvarum/S. eubayanus hybrid, and in S. cerevisiae wine strains erroneously labelled 'S. bayanus'. We made an inventory of their GAL genes using genomes that were either available publicly, re-sequenced by us, or assembled from public data and completed with targeted sequencing. In the S. eubayanus/S. uvarum CBS 380T hybrid, the GAL/MEL network is composed of genes from both parents: from S. uvarum, an otherwise complete set that lacks GAL4, and from S. eubayanus, a truncated version of GAL4 and an additional copy of GAL3 and GAL80. Similarly, two different truncated GAL4 alleles were found in S. cerevisiae wine strains EC1118 and LalvinQA23. The lack of GAL4 activity in these strains was corrected by introducing a full-length copy of S. cerevisiae GAL4 on a CEN4/ARS plasmid. Transformation with this plasmid restored galactose utilisation in Gal- strains, and melibiose fermentation in strain CBS 380T. The melibiose fermentation phenotype, formerly regarded as characteristic of S. uvarum, turned out to be widespread among Saccharomyces species.