Stress modulation as a means to improve yeasts for lignocellulose bioconversion

The second-generation (2G) fermentation environment for lignocellulose conversion presents unique challenges to the fermentative organism that do not necessarily exist in other industrial fermentations. While extreme osmotic, heat, and nutrient starvation stresses are observed in sugar- and starch-based fermentation environments, additional pre-treatment-derived inhibitor stress, potentially exacerbated by stresses such as pH and product tolerance, exist in the 2G environment. Furthermore, in a consolidated bioprocessing (CBP) context, the organism is also challenged to secrete enzymes that may themselves lead to unfolded protein response and other stresses. This review will discuss responses of the yeast Saccharomyces cerevisiae to 2G-specific stresses and stress modulation strategies that can be followed to improve yeasts for this application. We also explore published –omics data and discuss relevant rational engineering, reverse engineering, and adaptation strategies, with the view of identifying genes or alleles that will make positive contributions to the overall robustness of 2G industrial strains

A system based network approach to ethanol tolerance in Saccharomyces cerevisiae

BACKGROUND: Saccharomyces cerevisiae has been widely used for bio-ethanol production and development of rational genetic engineering strategies leading both to the improvement of productivity and ethanol tolerance is very important for cost-effective bio-ethanol production. Studies on the identification of the genes that are up- or down-regulated in the presence of ethanol indicated that the genes may be involved to protect the cells against ethanol stress, but not necessarily required for ethanol tolerance. RESULTS: In the present study, a novel network based approach was developed to identify candidate genes involved in ethanol tolerance. Protein-protein interaction (PPI) network associated with ethanol tolerance (tETN) was reconstructed by integrating PPI data with Gene Ontology (GO) terms. Modular analysis of the constructed networks revealed genes with no previously reported experimental evidence related to ethanol tolerance and resulted in the identification of 17 genes with previously unknown biological functions. We have randomly selected four of these genes and deletion strains of two genes (YDR307W and YHL042W) were found to exhibit improved tolerance to ethanol when compared to wild type strain. The genome-wide transcriptomic response of yeast cells to the deletions of YDR307W and YHL042W in the absence of ethanol revealed that the deletion of YDR307W and YHL042W genes resulted in the transcriptional re-programming of the metabolism resulting from a mis-perception of the nutritional environment. Yeast cells perceived an excess amount of glucose and a deficiency of methionine or sulfur in the absence of YDR307W and YHL042W, respectively, possibly resulting from a defect in the nutritional sensing and signaling or transport mechanisms. Mutations leading to an increase in ribosome biogenesis were found to be important for the improvement of ethanol tolerance. Modulations of chronological life span were also identified to contribute to ethanol tolerance in yeast. CONCLUSIONS: The system based network approach developed allows the identification of novel gene targets for improved ethanol tolerance and supports the highly complex nature of ethanol tolerance in yeast

Halomonas smyrnensis as a cell factory for co-production of PHB and levan

Levan is a fructan type polysaccharide that has long been considered as an industrially important biopolymer however its limited availability is mainly due to the bottlenecks associated with its large-scale production. To overcome such bottlenecks in the commercialization of this very promising polysaccharide, co-production of levan with polyhydroxyalkanoates (PHAs) by halophilic Halomonas smyrnensis cultures has been proposed in this study for the first time. After in silico and in vitro assessment of PHA accumulation, fermentation profiles for levan and PHA concentrations were obtained in the presence of sucrose and glucose and the PHA granules observed by TEM were found to be poly(3-hydroxybutyrate) (PHB) after detailed structural characterization by GC-MS, DSC, FTIR and NMR. Six nutrient limitation strategies based on nitrogen (N) and phosphorus (P) were tested but highest levan and PHB yields were obtained under unlimited conditions. H. smyrnensis is proved to co-produce PHB and levan while using inexpensive carbon sources which is a commercially successful microbial cell factory system showing a great potential in lowering manufacturing costs and aiming for a zero waste policy within the biorefinery concept. (C) 2018 Elsevier B.V. All rights reserved