Klebe et al. (1983) have described a method for transformation of bacteria and yeast that employs polythethylene glycol and does not require protoplast formation. However, the original version of this useful mcthod depends on freshly prepared competent cells and therefore does not allow longterm storage of competent cells for later use. We describe here modifications of the original procedure that yicld a simple and efficient method that allows storage of competent yeast at -70°C for several months without significant decrease in transformation efficiency. The procedure takes only -90 min and is applicable to species of various yeast genera (Table I ) . Among the advantages of the method is the possibility to transform yeast without delay whenever a relevant construct is available for transformation. Since aliquots of competent cells obtained from the same batch give highly reproducible numbers of transformants, the method is also useful in generating DNA libraries. The amount of library DNA that is required to obtain a convenient number of colonies per plate can be determined in pilot experiments.
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