Methanol-tolerant yeast for biofuel production

Candida boidinii isolates from olive curation water: a promising platform for methanol-based biomanufacturing

Methanol is a promising feedstock for biomanufacturing, but the efficiency of methanol-based bioprocesses is limited by the low rate of methanol utilization pathways and methanol toxicity. Yeast diversity is an attractive biological resource to develop efficient bioprocesses since any effort with strain improvement is more deserving if applied to innate robust strains with relevant catabolic and biosynthetic potential. The present study is in line with such rational and describes the isolation and molecular identification of seven isolates of the methylotrophic species Candida boidinii from waters derived from the traditional curation of olives, in different years, and from contaminated superficial soil near fuel stations. The yeast microbiota from those habitats was also characterized. The four C. boidinii isolates obtained from the curation of olives' water exhibited significantly higher maximum specific growth rates (range 0.15-0.19 h-1), compared with the three isolates obtained from the fuel contaminated soils (range 0.05-0.06 h-1) when grown on methanol as the sole C-source (1% (v/v), in shake flasks, at 30°C). The isolates exhibit significant robustness towards methanol toxicity that increases as the cultivation temperature decreases from 30°C to 25°C. The better methanol-based growth performance exhibited by C. boidinii isolates from olives´ soaking waters could not be essentially attributed to higher methanol tolerance. These methanol-efficient catabolizing isolates are proposed as a promising platform to develop methanol-based bioprocesses.

Med3-mediated NADPH generation to help Saccharomyces cerevisiae tolerate hyperosmotic stress

Hyperosmotic stress tolerance is crucial for Saccharomyces cerevisiae in producing value-added products from renewable feedstock. The limited understanding of its tolerance mechanism has impeded the application of these microbial cell factories. Previous studies have shown that Med3 plays a role in hyperosmotic stress in S. cerevisiae. However, the specific function of Med3 in hyperosmotic stress tolerance remains unclear. In this study, we showed that the deletion of the mediator Med3 impairs S. cerevisiae growth under hyperosmotic stress. Phenotypic analyses and yeast two-hybrid assays revealed that Med3 interacts with the transcription factor Stb5 to regulate the expression of the genes gnd1 and ald6, which are involved in NADPH production under hyperosmotic stress conditions. The deletion of med3 resulted in a decrease in intracellular NADPH content, leading to increased oxidative stress and elevated levels of intracellular reactive oxygen species under hyperosmotic stress, thereby impacting bud formation. These findings highlight the significant role of Med3 as a regulator in maintaining NADPH generation and redox homeostasis in S. cerevisiae during hyperosmotic stress.IMPORTANCEHyperosmotic stress tolerance in the host strain is a significant challenge for fermentation performance in industrial production. In this study, we showed that the S. cerevisiae mediator Med3 is essential for yeast growth under hyperosmotic conditions. Med3 interacts with the transcription factor Stb5 to regulate the expression of genes involved in the NADPH-generation system during hyperosmotic stress. Adequate NADPH ensures the timely removal of excess reactive oxygen species and supports bud formation under these conditions. This work highlights the crucial role of Med3 as a regulator in maintaining NADPH generation and redox homeostasis in S. cerevisiae during hyperosmotic stress.

Regulatory mechanisms underlying yeast chemical stress response and development of robust strains for bioproduction

Yeast is widely studied in producing biofuels and biochemicals using renewable biomass. Among various yeasts, Saccharomyces cerevisiae has been particularly recognized as an important yeast cell factory. However, economic bioproduction using S. cerevisiae is challenged by harsh environments during fermentation, among which inhibitory chemicals in the culture media or toxic products are common experiences. Understanding the stress-responsive mechanisms is conducive to developing robust yeast strains. Here, we review recent progress in mechanisms underlying yeast stress response, including regulation of cell wall integrity, membrane transport, antioxidative system, and gene transcription. We highlight epigenetic regulation of stress response and summarize manipulation of yeast stress tolerance for improved bioproduction. Prospects in the application of machine learning to improve production efficiency are also discussed.