Isolation of the Cryptococcus humicolus URA3 gene encoding orotidine-5'-phosphate decarboxylase and its use as a selective marker for transformation

Engineering Saccharomyces boulardii for Probiotic Supplementation of l-Ergothioneine

Saccharomyces boulardii, as a probiotic yeast, has shown great potential in regulating gut health and treating gastrointestinal diseases. Due to its unique antimicrobial and immune-regulating functions, it has become a significant subject of research in the field of probiotics. In this study, we aim to enhance the antioxidant properties of S. boulardii by producing l-ergothioneine (EGT). We first constructed a double knockout of ura3 and trp1 gene in S. boulardii to facilitate plasmid-based expressions. To further enable effective genome editing of S. boulardii, we implemented the PiggyBac system to transpose the heterologous gene expression cassettes into the chromosomes of S. boulardii. By using enhanced green fluorescent protein (EGFP) as the reporter gene, we achieved random chromosomal integration of EGFP expression cassette. By using PiggyBac transposon system, a great variety of EGT-producing strains was obtained, which is not possible for the conventional single target genome editing, and one best isolated top producer reached 17.50 mg/L EGT after 120 h cultivation. In summary, we have applied the PiggyBac transposon system to S. boulardii for the first time for genetic engineering. The engineered probiotic yeast S. boulardii has been endowed with new antioxidant properties and produces EGT. It has potential applications in developing novel therapeutics and dietary supplements for the prevention and treatment of gastrointestinal disorders.

Regulation of d-Aspartate Oxidase Gene Expression by Pyruvate Metabolism in the Yeast Cryptococcus humicola

d-Aspartate oxidase (DDO) is a peroxisomal flavoenzyme that catalyzes the oxidative deamination of acidic d-amino acids. In the yeast Cryptococcus humicola strain UJ1, the enzyme ChDDO is essential for d-Asp utilization and is expressed only in the presence of d-Asp. Pyruvate carboxylase (Pyc) catalyzes the conversion of pyruvate to oxaloacetate and is involved in the import and activation of certain peroxisomal flavoenzymes in yeasts. In this study, we analyzed the role of Pyc in the expression of ChDDO gene in C. humicola strain UJ1. PYC gene disruption (∆Chpyc1) in strain UJ1 resulted in growth retardation on glucose and NH₄Cl medium. The growth was restored by supplying oxaloacetate from l-Asp or α-ketoglutarate by a transaminase. On the other hand, the supply of oxaloacetate from d-Asp by ChDDO was not able to prevent growth retardation because of a significant decrease in ChDDO gene expression at the transcriptional level. The addition of pyruvate significantly decreased ChDDO gene transcription in the ∆Chpyc1 strain but increased the same in the wild-type strain, even though the intracellular pyruvate content was similar in both strains. These results suggest that ChDDO gene expression might be regulated by pyruvate metabolism, as well as by the presence of d-Asp.

Identification of an Acidic Amino Acid Permease Involved in d-Aspartate Uptake in the Yeast Cryptococcus humicola

d-aspartate oxidase (DDO) catalyzes the oxidative deamination of acidic d-amino acids, and its production is induced by d-Asp in several eukaryotes. The yeast Cryptococcus humicola strain UJ1 produces large amounts of DDO (ChDDO) only in the presence of d-Asp. In this study, we analyzed the relationship between d-Asp uptake by an amino acid permease (Aap) and the inducible expression of ChDDO. We identified two acidic Aap homologs, named “ChAap4 and ChAap5,” in the yeast genome sequence. ChAAP4 deletion resulted in partial growth defects on d-Asp as well as l-Asp, l-Glu, and l-Phe at pH 7, whereas ChAAP5 deletion caused partial growth defects on l-Phe and l-Lys, suggesting that ChAap4 might participate in d-Asp uptake as an acidic Aap. Interestingly, the growth of the Chaap4 strain on d- or l-Asp was completely abolished at pH 10, suggesting that ChAap4 is the only Aap responsible for d- and l-Asp uptake under high alkaline conditions. In addition, ChAAP4 deletion significantly decreased the induction of DDO activity and ChDDO transcription in the presence of d-Asp. This study revealed that d-Asp uptake by ChAap4 might be involved in the induction of ChDDO expression by d-Asp.

Development of an autonomously replicating linear vector of the yeast Cryptococcus humicola by using telomere-like sequence repeats

The yeast Cryptococcus humicola has several attractive properties for practical applications such as in bioremediation and as a source of industrially useful enzymes and compounds. We have developed an autonomously replicating vector of C. humicola to improve its properties. We initially tried to isolate an autonomously replicating sequence (ARS) from genomic DNA by transformation using a genomic DNA library. We obtained a candidate plasmid vector harboring an ARS that gave high transformation efficiency. Southern blot analysis of transformants revealed the autonomous replication of the introduced vector in some transformants. However, the vector was not only variously altered in length but also linearized. PCR analysis indicated that a telomere-like sequence repeat (TTAGGGGG)( n ) was added to the termini of linearized vector. Thus, we constructed an autonomously replicating linear vector having ten repeats of the telomere-like sequence at both ends. The vector transformed the yeast cells with high transformation efficiency (3230 CFU/μg of DNA), which was approximately 25-fold higher than that of a control vector lacking the repeats, and was autonomously replicated at a roughly constant size. The copy number was estimated to be less than one copy, and Ura(+) mitotic stability varied widely among the transformants and was related to plasmid segregation efficiency.

Physiological role of D-aspartate oxidase in the assimilation and detoxification of D-aspartate in the yeast Cryptococcus humicola

The physiological role of D-aspartate oxidase (ChDASPO) in the yeast Cryptococcus humicola was analysed through the growth characteristics of a ChDASPO gene-disrupted strain (daspoDelta) and the expression profile of ChDASPO on various combinations of carbon and nitrogen sources. The daspoDelta strain, constructed by homologous integration of the yeast URA3 marker, grew as well as the wild-type strain on ammonium chloride, L-aspartate or D-alanine as the sole nitrogen source. In contrast, the daspoDelta strain did not grow at all on D-aspartate, not only as the sole nitrogen source but also as the sole carbon source or as the sole nitrogen and carbon source, and grew more slowly than the wild-type strain on D-glutamate as the sole nitrogen source. In the wild-type strain, the induction of ChDASPO activity strictly depended on the presence of D-aspartate and was little affected by the co-presence of ammonium chloride, but it was significantly reduced by the co-presence of both glucose and ammonium chloride, which, however, did not abolish the induction, allowing considerable expression of ChDASPO. This expression pattern was consistent with that shown by Northern blot analysis. The daspoDelta strain was more sensitive than the wild-type to the growth retardation by acidic D-amino acids, but not to that by the corresponding L-isomers or D-alanine. These results clearly show that in the yeast, DASPO plays an essential role in the assimilation of D-aspartate and acts as a detoxifying agent for D-aspartate.