Effect of vector type, host strains and transcription terminator on heterologous gene expression in yeast

Propagation of an amplifiable recombinant plasmid in Saccharomyces cerevisiae: flow cytometry studies and segregated modeling

Efficient expression of a foreign protein product by the yeast Saccharomyces cerevisiae requires a stable recombinant vector present at a high number of copies per cell. A conditional centromere yeast plasmid was constructed which can be amplified to high copy number by a process of unequal partitioning at cell division, followed by selection for increased copy number. However, in the absence of selection pressure for plasmid amplification, copy number rapidly drops from 25 plasmids/cell to 6 plasmids/cell in less than 10 generations of growth. Copy number subsequently decreases from 6 plasmids/cell to 2 plasmids/cell over a span of 50 generations. A combination of flow cytometric measurement of copy number distributions and segregated mathematical modeling were applied to test the predictions of a conceptual model of conditional centromere plasmid propagation. Measured distributions of plasmid content displayed a significant subpopulation of cells with a copy number of 4-6, even in a population whose mean copy number was 13.5. This type of copy number distribution was reproduced by a mathematical model which assumes that a maximum of 4-6 centromere plasmids per cell can be stably partitioned at cell division. The model also reproduces the observed biphasic kinetics of plasmid number instability. The agreement between simulation and experimental results provides support for the proposed model and demonstrates the utility of the flow cytometry/segregated modeling approach for the study of multicopy recombinant vector propagation.

Genetic engineering of baker's and wine yeasts using formaldehyde hyperresistance-mediating plasmids

Yeast multi-copy vectors carrying the formaldehyde-resistance marker gene SFA have proved to be a valuable tool for research on industrially used strains of Saccharomyces cerevisiae. The genetics of these strains is often poorly understood, and for various reasons it is not possible to simply subject these strains to protocols of genetic engineering that have been established for laboratory strains of S. cerevisiae. We tested our vectors and protocols using 10 randomly picked baker's and wine yeasts all of which could be transformed by a simple protocol with vectors conferring hyperresistance to formaldehyde. The application of formaldehyde as a selecting agent also offers the advantage of its biodegradation to CO2 during fermentation, i.e., the selecting agent will be consumed and therefore its removal during down-stream processing is not necessary. Thus, this vector provides an expression system which is simple to apply and inexpensive to use.

Yeast 2 micron vectors replicate and undergo recombination in Torulaspora delbrueckii

In order to develop a procedure for transformation of the industrial yeast Torulaspora delbrueckii, we have constructed a set of recombinant plasmids carrying Saccharomyces cerevisiae ARS and 2 microns origin of replication and kanamycin-G418 resistance gene of Tn903(601) as a selective marker. In this paper we show that S. cerevisiae ARS vectors can replicate autonomously and that vectors bearing the whole S. cerevisiae 2 microns sequence yield stable transformants. We also present evidence to show that 2 microns vectors undergo an FLP-mediated inter- and intramolecular recombination, which suggests that T. delbrueckii can support the amplification and partition mechanisms of these plasmids.