Yeast Expression Platform - Do The Various Yeasts Perform in An Identical Way?

Do The Various Yeasts Perform in An Identical Way?

The answer is “no”. They differ in productivity and with respect to their capabilities to secrete, to process and to modify proteins in particular examples. First we explain how a yeast becomes a producer of foreign proteins.

Suitable yeast strains are transformed by a vector, a so-called plasmid that contains all necessary genetic elements for recognition of a transformed strain and the genetic advice for the production of a protein. The elements are summarized in the following:

1. A selection marker, required to select a transformed strain from an untransformed background – this can be done if for instance such an element enables a deficient strain to grow under culturing conditions void of a certain indispensable compound like a particular amino acid that cannot be produced by the deficient strain.
2. Certain elements to propagate and to target the foreign DNA to the chromosome of the yeast (ARS and/or rDNA sequence).
3. A segment responsible for the production of the desired protein compound a so-called expression cassette. Such a cassette is made up by a sequence of regulatory elements, a promoter that controls, how much and under which circumstances a following gene sequence is transcribed and as a consequence how much protein is eventually made. This means that the segment following the promoter is variable depending on the desired product – it could be for instance a sequence determining the amino acids for insulin, for hepatitis B vaccine or for IFN alpha-2a. The expression cassette is terminated by a following terminator sequence that provides a proper stop of the transcription. The promoter elements of the H. polymorpha system are derived from genes that are highly expressed. Some of them are not only very strong, but can also be regulated by certain addition of carbon sources like sugar, methanol or glycerol. However, most of them can only be recognized by a single yeast species.

However, since the yeasts differ in their characteristics to produce a certain protein it cannot be excluded at the beginning of a development that a selected yeast will not be able to produce the desired compound at all. This in turn can lead to costly time-consuming failures. It is therefore advisable to assess several yeast platforms in parallel for their capabilities to produce such a compound. Therefore, a plasmid system was developed that can be targeted in functional form to all yeast in parallel. The basic design of this vector system, designated CoMed, is shown in the following figure. It is composed in modular way of element for selection, a “universal” targeting sequence that is present in all yeasts (the rDNA) and it contains within the expression cassette a promoter that is active in all yeasts (Fig. 1).