Abstract

Adipic acid is used in the production of a range of important products, notably nylon, but its production using traditional means relies on petroleum precursors and is environmentally damaging. The production of adipic acid from renewable sources using microbial cell factories has generated significant interest yet fails to compete economically with its petro-based alternative, partly due to the large expense required for post-fermentation processing. Using a low pH fermentation strategy, downstream acidification costs can be decreased, which will help with raising the economic viability of bio-based adipic acid.
We screened a large collection of tentatively acid tolerant strains of Saccharomyces cerevisiae to assess adipic acid tolerance and later characterised top adipic acid tolerant strains for suitability as adipic acid production hosts. Top strains were mostly diploid and had growth yields in adipic acid roughly 5x higher than the non-tolerant strain CEN.PK113-7D. Furthermore, we show that adipic acid tolerance is an inducible phenotype -- pre-adaptation in adipic acid reduces latency upon subsequent exposure to adipic acid. Next, we investigated transcriptional changes by way of RNAseq. Gene content was largely the same across 4 different strains of S. cerevisiae. Exposure to adipic acid caused cell-wide transcriptional changes in both tolerant and non-tolerant strains. Differences in gene expression, especially in stress response, provided an explanation for the adipic acid tolerant phenotype but, for a more complete picture, more work is still required. Finally, as a proof of concept for the validity of acid tolerant production strains, we expressed aroZ, necessary for the first step in adipic acid production from the shikimate pathway, in an adipic acid tolerant strain and showed increased PCA, an adipic acid precursor, accumulation when grown at low pH and under adipic acid stress compared to a similarly engineered adipic acid-sensitive CEN.PK113-7D. Our results emphasize the utility of adipic acid tolerant strains for the production of adipic acid. Other carboxylic acid may also benefit from similarly acid tolerant production strains, which could help improve bio-based alternatives in the chemicals industry and reduce our reliance on petroleum-based products.