Current environmental, economic and security issues demand a switch from the use of fossil materials for energy generation. Lignocellulosic biomass offers a more reliable, sustainable and environmentally friendly alternative. One of the limitations in the use of lignocelluloses is the relatively less efficient fermentation of pentose sugars (xylose and arabinose) by the baker’s yeast Saccharomyces cerevisiae into ethanol. Yeast has evolved to prefer hexose sugars such as glucose for growth and fermentation over pentose sugars. One reason could be xylose uptake into the yeast cells. Yeast lacks specific xylose transporters – using hexose transporters instead that have substantially higher affinity for glucose compared to xylose. 

In our current study, we looked into the galactose transport to study possible improvements to pentose sugar uptake. Gal2p is a carrier that transports sugars into yeast cells through facilitated diffusion. Another protein in the galactose pathway is galactokinase (Gal1p). We over-expressed Gal2 alone and then Gal1+Gal2 together in a yeast strain that lacked major hexose transporters. Over-expression of Gal2 gene alone or Gal1+Gal2 genes together restores the ability of this strain to grow on glucose and thereby affords us the ability to compare Gal2 alone with Gal1+Gal2. Our results show that strains carrying Gal1+Gal2 have higher xylose and arabinose uptake ability compared to strains carrying Gal2 alone. For comparison we also carried out uptake and fermentation studies of strains over-expressing HXT-7 and GXF1.