The evolution and role of the periplasmic asparaginase Asp3 in yeast.

The study of nitrogen assimilation in yeast is of interest from genetic, evolutionary, and biotechnological perspectives. Over the course of evolution, yeasts have developed sophisticated control mechanisms to regulate nitrogen metabolism, with domesticated lineages sometimes displaying particular specialisation. The focus of this study was on assimilation of asparagine, which is a significant nutritional source for some alcoholic fermentations. We were particularly interested in ASP3, which encodes a periplasmic asparaginase and that was proposed to have been acquired relatively recently in S. cerevisiae by horizontal gene transfer. We examined 1680 S. cerevisiae genome assemblies to evaluate the distribution and evolutionary trajectory of ASP3. Our findings suggest an alternative hypothesis that ASP3 is an ancient Saccharomyces gene that has generally been lost over the course of evolution but has been retained in certain fermentative environments. As asparagine is the major nitrogen source in apple juice, we explored whether the presence of ASP3 would confer a growth advantage. Interestingly, we found that although ASP3 enhances growth when asparagine is the sole nitrogen source, the same effect is not seen in apple juice. These data indicate that growth in pure culture may not reflect the original selective environment for ASP3+ strains and highlight the role that complex regulation may play in optimising nitrogen assimilation in yeasts.