The genetic improvement of brewing yeast
Brewing yeast strains are usually aneuploid or polyploid with no true mating type. Thus many of the techniques which can be used to genetically modify laboratory yeast strains cannot be applied to them. This study was aimed at developing the technology for the genetic modification of brewing yeasts towards producing genetically improved brewing strains, suitable for use in large scale beer production. A system has been developed which can produce brewing yeast transformants with high genetic stability and unimpaired brewing performance. Such transformants contain only the required extra genetic material and no superfluous vector DNA. This system has also been demonstrated to have potential for the genetic labelling of yeast strains as a means of accurate identification which could also be used towards the patent protection of commercially important yeast strains. _ A method for the stabilisation of heterologous proteins produced in yeast has been investigated. The hypothesis that the glycosylation of heterologous proteins can improve their stability in yeast was tested by mutating the Escherichia coli ß-lactamase gene to produce N-linked glycosylation substrates. Two of the modified proteins were glycosylated when expressed in yeast, one of which had improved thermal stability. A wheat 11-amylase gene yeast expression cassette has been constructed using the wheat secretion signal and yeast gene control sequences. Expression of the cassette in yeast results in efficient starch-degrading activity both in liquid and on solid media. The enzyme is efficíently secreted and approximately 90% of the activity is found in yeast culture supernatants.