ABSTRACT
This chapter aims to give a brief description of some aspects of the yeast transformation system and to discuss its impact in the area of gene isolation and gene expression. The yeast transformation procedure is based on the coprecipitation of calcium-treated yeast protoplasts and transforming DNA by polyethylene glycol 4000. The crucial step in the transformation protocol is the preparation of spheroplasts, which is exclusively done with the help of cell wall degrading enzymes of different sources. Depending on the mode of replication in yeast, they can be divided into two classes: integrating vectors and extrachromosomally replicating vectors. Since the yeast transformation protocol is basically derived from a yeast protoplast fusion protocol, it is generally assumed that fusionlike conditions are also conditions that favor DNA uptake. The chapter discusses the potential of the yeast cloning system for isolating nonyeast genes via heterologous complementation. Vectors with yeast replication origins can autonomously replicate and have no need for integration.
