Saccaromyces cerevisiae

Saccaromyces cerevisiae represents an eucariotic microorganism. Yeast has good genetic characteristics. Like other microorganisms, they grow well and have a haploid DNA genome, which is 3.5 larger than that of E. coli. In spite of a large genome, yeast has certain qualities, which are characteristic of higher eucariots. Yeast is easy to cultivate. Yeast reproduces in a mitotic way. Its quantity in the growth medium doubles every 90 minutes.

Yeast may exist in a diploidic or haploidic state. Genome of a haploidic yeast cell is 15 megabases in size and has 16 linear chromosomes, the sizes of which vary from 200 to 2200 kb. The largest yeast chromosome is 100 times smaller than an average mammal chromosome. Yeast is also characterized by high transformation effectiveness.

Yeast expression system for recombinant protein synthesis

Expression systems of various kinds of yeast have been created for protein synthesis. S. cerevisiae are widely recognized as safe microorganisms and represent eucariots, which are best researched from the points of view of genetics, molecular biology, and physiology. Vectors used for yeast transformation usually have sequences of bacterial or yeast origin. The bacterial part allows for use of bacteria for construction of plasmids. It contains bacteria plasmid replication start area ori and the gene, determining resistance to an antibiotic (allowing for selection of transformants). The part of yeast origin has genes for selection of yeast transformants, sequences ensuring replication and distribution (or integration), gene transcription promotors and terminators. Expression of cloned genes requires strong and adjustable promotors. Promotors of their glycolysis enzyme genes are often used in S. cerevisiae. One part of the hybrid promotors determine strong expression (e.g., PYK1 – pyruvatkinasis promotor), while the other allows for its regulation (GAL10 – galactose-induced UDP-galactose epimerasis gene promoter). Possibility to induce expression of a gene at the right moment is important, because strong promoters determine a very intense protein synthesis, which may impair viability of cells (the same protein may be toxic). For termination of transcription, terminatory areas of genes are used (e.g., PGK1 – 3-phosphoglicerate kinasis).

Application of P. pastoris expression system for synthesis of viral proteins is becoming more and more widespread lately. The distinctive feature of this yeast is its ability to grow using methanol as the only source of energy and carbon. All post-translation modifications, which are possible in higher eucariotic organisms, are also possible in P.pastoris yeast.