Shi-An Wang

Surprisingly diverged populations of Saccharomyces cerevisiae in natural environments remote from human activity

The budding yeast, Saccharomyces cerevisiae, is a leading system in genetics, genomics and molecular biology and is becoming a powerful tool to illuminate ecological and evolutionary principles. However, little is known of the ecology and population structure of this species in nature. Here, we present a field survey of this yeast at an unprecedented scale and have performed population genetics analysis of Chinese wild isolates with different ecological and geographical origins. We also included a set of worldwide isolates that represent the maximum genetic variation of S. cerevisiae documented so far. We clearly show that S. cerevisiae is a ubiquitous species in nature, occurring in highly diversified substrates from human‐associated environments as well as habitats remote from human activity. Chinese isolates of S. cerevisiae exhibited strong population structure with nearly double the combined genetic variation of isolates from the rest of the world. We identified eight new distinct wild lineages (CHN I–VIII) from a set of 99 characterized Chinese isolates. Isolates from primeval forests occur in ancient and significantly diverged basal lineages, while those from human‐associated environments generally cluster in less differentiated domestic or mosaic groups. Basal lineages from primeval forests are usually inbred, exhibit lineage‐specific karyotypes and are partially reproductively isolated. Our results suggest that greatly diverged populations of wild S. cerevisiae exist independently of and predate domesticated isolates. We find that China harbours a reservoir of natural genetic variation of S. cerevisiae and perhaps gives an indication of the origin of the species.

Saccharomyces arboricolus sp. nov., a yeast species from tree bark

Three ascomycetous yeast strains, H-6(T), ZX-15 and ZX-20, isolated from the bark of two tree species of the family Fagaceae collected from different regions of China, formed unconjugated and persistent asci containing two to four globose ascospores. 26S rDNA D1/D2 domain and internal transcribed spacer (ITS) region (including 5.8S rDNA) sequence analysis showed that they were closely related to the currently accepted Saccharomyces species with strong support. Comparisons of the rDNA sequences, electrophoretic karyotypes and physiological characters indicated that the three strains represent a novel species in the genus Saccharomyces. The name Saccharomyces arboricolus sp. nov. was proposed for the novel species, with H-6(T) (=AS 2.3317(T)=CBS 10644(T)) isolated from the bark of Quercus fabri as the type strain.

Invertase SUC2 Is the key hydrolase for inulin degradation in Saccharomyces cerevisiae.

ABSTRACT Specific Saccharomyces cerevisiae strains were recently found to be capable of efficiently utilizing inulin, but genetic mechanisms of inulin hydrolysis in yeast remain unknown. Here we report functional characteristics of invertase SUC2 from strain JZ1C and demonstrate that SUC2 is the key enzyme responsible for inulin metabolism in S. cerevisiae.

Improved ethanol fermentation by heterologous endoinulinase and inherent invertase from inulin by Saccharomyces cerevisiae

It is hypothesized that introduction of an endoinulinase gene into Saccharomyces cerevisiae will improve its inulin utilization and ethanol fermentation through collaboration between the heterologous endoinulinase and the inherent invertase SUC2. The aim of this work was to test the hypothesis by introducing the endoinulinase gene inuA from Aspergillus niger into S. cerevisiae. The results showed that heterologous inuA expressed in S. cerevisiae selectively digested long chains of inulin into short fructooligosaccharides and parts of these fructooligosaccharides could be efficiently utilized by the yeast. This study demonstrated that collaboration between heterologous endoinulinase and inherent invertase improved inulin degradation and ethanol fermentation in S. cerevisiae.

Rapid Differentiation of Phenotypically Similar Yeast Species by Single-Strand Conformation Polymorphism Analysis of Ribosomal DNA

ABSTRACT Single-strand conformation polymorphism (SSCP) analysis of ribosomal DNA (rDNA) was investigated for rapid differentiation of phenotypically similar yeast species. Sensitive tests indicated that some yeast strains with one, most strains with two, and all strains with three or more nucleotide differences in the internal transcribed spacer 1 (ITS1) or ITS2 region could be distinguished by PCR SSCP analysis. The discriminative power of SSCP in yeast species differentiation was demonstrated by comparative studies of representative groups of yeast species from ascomycetes and basidiomycetes, including Saccharomyces species, medically important Candida species, and phylloplane basidiomycetous yeast species. Though the species within each group selected are closely related and have relatively similar rDNA sequences, they were clearly differentiated by PCR-SSCP analysis of the ITS1 region, given the amplified fragments were less than 350 bp in sizes. By using SSCP analysis for rapid screening of yeast strains with different rDNA sequences, species diversity existing in a large collection of yeast strains from natural sources was effectively and thoroughly investigated with substantially reduced time and cost in subsequent DNA sequencing.

Candida laoshanensis sp. nov. and Candida qingdaonensis sp. nov., anamorphic, ascomycetous yeast species isolated from decayed wood

During a study of newly isolated yeast strains utilizing d-xylose as sole carbon source, eight strains, isolated from decayed wood, were found to represent two novel anamorphic, ascomycetous yeast species based on sequence analysis of the 26S rDNA D1/D2 domain and internal transcribed spacer region, and phenotypic characterization. The names Candida laoshanensis sp. nov. (type strain MLRW 6-2(T)=AS 2.4030(T)=CBS 11389(T)) and Candida qingdaonensis sp. nov. (type strain MLRW 7-1(T)=AS 2.4031(T)=CBS 11390(T)) are proposed for these two novel species; the closest relatives of the two novel species are Candida pomicola and Candida marilandica, respectively.

A one-step bioprocess for production of high-content fructo-oligosaccharides from inulin by yeast

Commercial fructo-oligosaccharides (FOS) are predominantly produced from sucrose by transfructosylation process that presents a maximum theoretical yield below 0.60gFOSgSucrose(-1). To obtain high-content FOS, costly purification is generally employed. Additionally, high-content FOS can be produced from inulin by using endo-inulinases. However, commercial endo-inulinases have not been extensively used in scale-up production of FOS. In the present study, a one-step bioprocess that integrated endo-inulinase production, FOS fermentation, and non-FOS sugars removal into one reactor was proposed to produce high-content FOS from inulin. The bioprocess was implemented by a recombinant yeast strain JZHΔS-TSC, in which a heterologous endo-inulinase gene was expressed and the inherent invertase gene SUC2 was disrupted. FOS fermentation at 40°C from 200g/L chicory inulin presented the maximun titer, yield, and productivity of 180.2±0.8g/L, 0.9gFOSgInulin(-1), and 7.51±0.03g/L/h, respectively. This study demonstrated that the one-step bioprocess was simple and highly efficient.

Starmerella orientalis f.a., sp. nov., an ascomycetous yeast species isolated from flowers.

Four strains of a novel ascomycetous yeast species were isolated from flowers in Iran and China. Phylogenetic analysis of the sequences of the ITS region (including 5.8S rRNA gene) and the LSU rRNA gene D1/D2 domains indicated that these strains belong to the Starmerella clade and show divergence from previously described species in this clade. Growth reactions on carbon and nitrogen sources were similar to those observed in related species of the Starmerella clade. Sexual reproduction was not observed after mating tests on different sporulation media. Based on physiological characteristics and phylogeny of rRNA gene sequences, the novel species is most closely related to Candida (iter. nom. Starmerella) powellii and Candida (iter. nom. Starmerella) floricola. It is therefore assigned to the genus Starmerella and described as Starmerella orientalis f.a., sp. nov. The type strain is SAM09T ( = IBRC-M 30204T = CBS 14142T). The MycoBank accession number is MB 814379.

Stress Tolerance Variations in Saccharomyces cerevisiae Strains from Diverse Ecological Sources and Geographical Locations

The budding yeast Saccharomyces cerevisiae is a platform organism for bioethanol production from various feedstocks and robust strains are desirable for efficient fermentation because yeast cells inevitably encounter stressors during the process. Recently, diverse S. cerevisiae lineages were identified, which provided novel resources for understanding stress tolerance variations and related shaping factors in the yeast. This study characterized the tolerance of diverse S. cerevisiae strains to the stressors of high ethanol concentrations, temperature shocks, and osmotic stress. The results showed that the isolates from human-associated environments overall presented a higher level of stress tolerance compared with those from forests spared anthropogenic influences. Statistical analyses indicated that the variations of stress tolerance were significantly correlated with both ecological sources and geographical locations of the strains. This study provides guidelines for selection of robust S. cerevisiae strains for bioethanol production from nature.