Variation in frequency of transformation by plasmid YRp7 in Saccharomyces cerevisiae

Hybrid plasmid YRp7 (Escherichia coli plasmid pBR322 carrying a 1.4-kb yeast fragment containing the TRP1 gene) transforms two related haploid yeast strains (trp1) to Trp+ at frequencies per microgram DNA varying by two orders of magnitude. The diploid cross of these two strains is transformed at a frequency less than that of the low-frequency parent, indicating that high-frequency transformation is inherited in a recessive fashion. Segregant strains of two tetrads of this cross showed intermediate levels of transformability, suggesting the polygenic inheritance of transformation frequency. Levels of variation are changed extensively when frequencies are expressed as transformants per regenerated spheroplast colony, although the inheritance of higher frequency remains clearly recessive. Storage of spheroplast-DNA preparations at 4 degrees C increases the yield of transformants with some strains.