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Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitute 1% of all described fungal species. Yeasts are unicellular organisms that evolved from multicellular ancestors, with some species having the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae. Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4 µm in diameter, although some yeasts can grow to 40 µm in size. Most yeasts reproduce asexually by mitosis, and many do so by the asymmetric division process known as budding. With their single-celled growth habit, yeasts can be contrasted with molds, which grow hyphae. Fung

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Production of a thermostable fumarase in Saccharomyces cerevisiae for the bioconversion of fumarate to L-malate

Coralie Signorell

The gene encoding fumarase (fum) from Thermus thermophilus was expressed in yeast Saccharomyces cerevisiae. The recombinant cells were heated at 70°C to inactivate indigenous enzymes and used for the bioconversion of fumaric acid to L-malic acid. By heating the host cells at 70°C, substrate is able to go across the heat-damaged membrane of the microorganism and a desired product can be formed. This new concept, called Synthetic Metabolic Engineering (SME), has already been applied successfully in Escherichia coli. Unfortunately, E. coli membrane is weakened too much during the heat treatment and enzyme leakage appears. The surface structure of yeast is more rigid than that of E. coli and this might be taken as an advantage for application of SME. When continuous or repeated-batch reaction is carried out, enzyme leakage becomes a major drawback. It is assumed that yeast cells could overcome this problem by retaining more enzymes in the cell during and after the heat treatment. In order to prove this hypothesis, a thermophilic fumarase (FUM) was over-expressed in two hosts, S. cerevisiae as well as E. coli. fum was first modified to be over-expressed in yeast cells and FUM was successfully produced in yeasts. Optimization of SME techniques was carried out for yeast cells. Then, enzyme activity and enzyme leakage was investigated for both strains. E. coli showed high level of FUM expression, though considerable amount of enzyme leaked to supernatant. On the other hand, even though the level of FUM expression in S. cerevisiae was low, yeast cells overcome leakage problem and are re-usable. This study showed the first trial of SME in yeast cells and possibility of utilization of yeast as a host strain for SME.

2011

Localization of Sir2p: the nucleolus as a compartment for silent information regulators

Thierry Laroche

In wild-type budding yeast strains, the proteins encoded by SIR3, SIR4 and RAP1 co-localize with telomeric DNA in a limited number of foci in interphase nuclei. Immunostaining of Sir2p shows that in addition to a punctate staining that coincides with Rap1 foci, Sir2p localizes to a subdomain of the nucleolus. The presence of Sir2p at both the spacer of the rDNA repeat and at telomeres is confirmed by formaldehyde cross-linking and immunoprecipitation with anti-Sir2p antibodies. In strains lacking Sir4p, Sir3p becomes concentrated in the nucleolus, by a pathway requiring SIR2 and UTH4, a gene that regulates life span in yeast. The unexpected nucleolar localization of Sir2p and Sir3p correlates with observed effects of sir mutations on rDNA stability and yeast longevity, defining a new site of action for silent information regulatory factors.

1997

Ribonuclease H(70) is a component of the yeast nuclear scaffold

Thierry Laroche

We have used monospecific antibodies against three ribonuclease H enzymes of Saccharomyces cerevisiae to investigate their intracellular localization. Fractionation experiments, as well as immunocytochemical staining, revealed a predominantly cytoplasmic localization of the RNase H proteins of 42,000 and 70,000 Mr, whereas that of 55,000 Mr showed equal distribution between nuclei and cytoplasm. The nuclear moiety of ribonuclease H(70) was found to be a part of the yeast nuclear scaffold, as investigated by immunoblotting and antibody inhibition experiments. The 42,000 and 55,000 Mr enzymes, on the other hand, are not scaffold-associated. We conclude that RNase H(70) is part of the nuclear substructure of yeast that was previously found to maintain specific interactions with yeast chromosomal origins of replication (ARS elements).

1990