Yeasts have evolved divergent enzyme strategies to deconstruct and metabolize xylan

Abstract

Together with bacteria and fungi, yeasts actively take part in the global carbon cycle. Over a hundred yeast species have been shown to grow on the major plant polysaccharide xylan, which requires an arsenal of carbohydrate active enzymes. However, which enzymatic strategies yeasts use to deconstruct xylan and what specific biological roles they play in its conversion remain unclear. In fact, genome analyses reveal that many xylan-metabolizing yeasts lack expected xylanolytic enzymes. Guided by bioinformatics, we have here selected three xylan-metabolizing ascomycetous yeasts for in-depth characterization of growth behavior and xylanolytic enzymes. The savanna soil yeastBlastobotrys mokoenaiidisplays superior growth on xylan thanks to an efficient secreted glycoside hydrolase family 11 (GH11) xylanase; solving its crystal structure revealed a high similarity to xylanases from filamentous fungi. The termite gut-associatedScheffersomyces lignosusin contrast grows more slowly and its xylanase activity was found to be mainly cell surface-associated. The wood-isolatedWickerhamomyces canadensissurprisingly could not utilize xylan as the sole carbon source without adding xylooligosaccharides, exogenous xylanases or even by co-culturing withB. mokoenaii, suggesting thatW. canadensisrelies on initial xylan hydrolysis by neighboring cells. Furthermore, our characterization of a novelW. canadensisGH5 subfamily 49 (GH5_49) xylanase represents the first demonstrated activity in this subfamily. Our collective results provide new information on the variable xylanolytic systems evolved by yeasts and their potential roles in natural carbohydrate conversion.