An innovative protein expression system using RNA polymerase I for large-scale screening of high-nucleic-acid content Saccharomyces cerevisiae strains

Transcriptome analysis of Kluyveromyces marxianus under succinic acid stress and development of robust strains

Kluyveromyces marxianus has become an attractive non-conventional yeast cell factory due to its advantageous properties such as high thermal tolerance and rapid growth. Succinic acid (SA) is an important platform molecule that has been applied in various industries such as food, material, cosmetics, and pharmaceuticals. SA bioproduction may be compromised by its toxicity. Besides, metabolite-responsive promoters are known to be important for dynamic control of gene transcription. Therefore, studies on global gene transcription under various SA concentrations are of great importance. Here, comparative transcriptome changes of K. marxianus exposed to various concentrations of SA were analyzed. Enrichment and analysis of gene clusters revealed repression of the tricarboxylic acid cycle and glyoxylate cycle, also activation of the glycolysis pathway and genes related to ergosterol synthesis. Based on the analyses, potential SA-responsive promoters were investigated, among which the promoter strength of IMTCP2 and KLMA_50231 increased 43.4% and 154.7% in response to 15 g/L SA. In addition, overexpression of the transcription factors Gcr1, Upc2, and Ndt80 significantly increased growth under SA stress. Our results benefit understanding SA toxicity mechanisms and the development of robust yeast for organic acid production. KEY POINTS: • Global gene transcription of K. marxianus is changed by succinic acid (SA) • Promoter activities of IMTCP2 and KLMA_50123 are regulated by SA • Overexpression of Gcr1, Upc2, and Ndt80 enhanced SA tolerance.

Promoting Nucleic Acid Synthesis in Saccharomyces cerevisiae through Enhanced Expression of Rrn7p, Rrn11p, IMPDH, and Pho84p

Saccharomyces cerevisiae has emerged as a preferred source for industrial production of ribonucleic acids (RNAs) and their derivatives, which find wide applications in the food and pharmaceutical sectors. In this study, we employed a modified RNA polymerase I-mediated green fluorescent protein expression system, previously developed by our team, to screen and identify an industrial S. cerevisiae strain with an impressive 18.2% increase in the RNA content. Transcriptome analysis revealed heightened activity of genes and pathways associated with rRNA transcription, purine metabolism, and phosphate transport in the high nucleic acid content mutant strains. Our findings highlighted the crucial role of the transcription factor Sfp1p in enhancing the expression of two key components of the transcription initiation factor complex, Rrn7p and Rrn11p, thereby promoting rRNA synthesis. Moreover, elevated expression of 5'-inosine monophosphate dehydrogenases, regardless of the specific isoform (IMD2, 3, or 4), resulted in increased rRNA synthesis through heightened GTP levels. Additionally, exogenous phosphate application, coupled with overexpression of the phosphate transporter PHO84, led to a 61.4% boost in the RNA yield, reaching 2050.4 mg/L. This comprehensive study provides valuable insights into the mechanism of RNA synthesis and serves as a reference for augmenting RNA production in the food industry.

Enhanced Ribonucleic Acid Production by High-Throughput Screening Based on Fluorescence Activation and Transcriptomic-Guided Fermentation Optimization in Saccharomyces cerevisiae

Currently, the primary source of ribonucleic acids (RNAs), which is used as a flavor enhancer and nutritional supplement in the food manufacturing and processing industries, for large-scale industrial production is yeast, where the challenge is to optimize the cellular RNA content. Here, we developed and screened yeast strains yielding abundant RNAs via various methods. The novel Saccharomyces cerevisiae strain H1 with a 45.1% higher cellular RNA content than its FX-2 parent was successfully generated. Comparative transcriptomic analysis elucidated the molecular mechanisms underlying RNA accumulation in H1. Upregulation of genes encoding the hexose monophosphate and sulfur-containing amino acid biosynthesis pathways promoted RNA accumulation in the yeast, particularly in the presence of glucose as the sole carbon source. Feeding methionine into the bioreactor resulted in 145.2 mg/g dry cell weight and 9.6 g/L of cellular RNA content, which is the highest volumetric productivity of RNAs achieved in S. cerevisiae. This strategy of breeding S. cerevisiae strain with a higher capacity of accumulating abundant RNAs did not employ any genetic modification and thus will be favored by the food industry.

Fed-Batch Fermentation of Saccharomyces pastorianus with High Ribonucleic Acid Yield

(1) Background: The degradation products of ribonucleic acid (RNA)are widely used in the food and pharmaceutical industry for their flavoring and nutritional enhancement functions. Yeast is the main source for commercial RNA production, and an efficient strain is the key to reducing production costs; (2) Methods: A mutant Saccharomyces pastorianus G03H8 with a high RNA yield was developed via ARTP mutagenesis and fed-batch fermentation was applied to optimize production capacity. Genome sequencing analysis was used to reveal the underlying mechanism of higher RNA production genetic differences in the preferred mutant; (3) Results: Compared with the highest RNA content of the mutant strain, G03H8 increased by 40% compared with the parental strain G03 after response surface model optimization. Meanwhile, in fed-batch fermentation, G03H8′s dry cell weight (DCW) reached 60.58 g/L in 5 L fermenter by molasses flowing and RNA production reached up to 3.58 g/L. Genome sequencing showed that the ribosome biogenesis, yeast meiosis, RNA transport, and longevity regulating pathway were closely related to the metabolism of high RNA production; (4) Conclusion: S. pastorianus G03H8 was developed for RNA production and had the potential to greatly reduce the cost of RNA production and shorten the fermentation cycle. This work lays the foundation for efficient RNA content using S. pastorianus.

Analysis of the molecular basis of Saccharomyces cerevisiae mutant with high nucleic acid content by comparative transcriptomics

Ribonucleic acid (RNA) and its degradation products are important functional components widely used in the food industry. Transcription analysis was used to explore the genetic mechanism underlying nucleic acid synthesis in the chemical mutant Saccharomyces cerevisiae strain BY23-195 with high nucleic acid content. Results showed that ribosome biogenesis, meiosis, RNA transport, mitogen-activated protein kinase (MAPK) signaling pathway, tryptophan metabolism, carbon metabolism, and longevity regulating pathway are closely related to the high nucleic acid metabolism of S. cerevisiae. Fourteen most promising genes were selected to evaluate the effect of single-gene deletion or overexpression on the RNA synthesis of S. cerevisiae. Compared with the RNA content of the parent strain BY23, that of mutant strains BY23-HXT1, BY23-ΔGSP2 and BY23-ΔCTT1 increased by 8.19%, 11.60% and 14.00%, respectively. The possible reason why HXT1, GSP2, and CTT1 affect RNA content is by regulating cell fitness. This work was the first to report that regulating the transcription of HXT1, GSP2, and CTT1 could increase the RNA content of S. cerevisiae. This work also provides valuable knowledge on the genetic mechanism of high nucleic acid synthesis in S. cerevisiae and new strategies for increasing its RNA content.