The evolution of the GALactose utilization pathway in budding yeasts

Abstract

The Leloir galactose utilization or GAL pathway of budding yeasts, including that of the baker's yeast Saccharomyces cerevisiae and the opportunistic human pathogen Candida albicans, breaks down the sugar galactose for energy and biomass production. The GAL pathway has long served as a model system for understanding how eukaryotic metabolic pathways, including their modes of regulation, evolve. More recently, the physical linkage of the structural genes GAL1, GAL7, and GAL10 in diverse budding yeast genomes has been used as a model for understanding the evolution of gene clustering. In this review, we summarize exciting recent work on three different aspects of this iconic pathway's evolution: gene cluster organization, GAL gene regulation, and the population genetics of the GAL pathway.

Keywords: fungi; galactose; gene cluster; gene regulation; horizontal gene transfer; polymorphism; regulatory rewiring.

Figure 1.

Comparison of genomic organization, function, and mode of regulation of the GAL pathway in the model organisms C. albicans (left panel) and S. cerevisiae (right panel). Although GAL102 and ORF-X are nested within the GAL gene cluster in C. albicans, their functions are not known to be related to galactose assimilation. Information displayed in the figure based on: [3,8,21].

Figure 2.

Genomic organization of GAL gene clusters in different budding yeast major clades. Note the differing patterns of presence / absence of GAL pathway genes in between major clades (indicated by the large colored rectangles). Lines correspond to homologs; gene box colors correspond to different functional categories of genes. Information displayed in the figure based on: [11,21,23].

Figure 3.

Gene presence and absence of key transcription factors involved in GAL pathway regulation, as well as variation in the presence and absence of their transcription factor-binding site sequence motifs across budding yeasts. Clustering is defined as GAL1, GAL7, and GAL10 having 0–5 ORFs between them. Domain I of GAL4 encodes amino acids 1–76 and Domain V amino acids 767–881 of the S. cerevisiae protein [30,31]. Information displayed in the figure based on [11,17,18].