The invention provides a universal 
yeast donor strain that contains a conditional 
centromere and a URA3 
allele on every 
chromosome. This strain was constructed in four rounds of crosses of individual conditional 
chromosome strains using a novel tetrad-based screen to identify segregants in which all marked chromosomes were contained in the same 
spore. The invention also provides an improved high efficiency method to transfer extrachromosomal genetic material such as 
plasmid DNA into any 
Saccharomyces strain for use with the current 
gene disruption libraries. The method of transfer is 
mating-based method which uses a kar1 
plasmid donor strain that can initiate 
mating but cannot form a diploid and allows 
plasmid transfer (plasmoduction) between nuclei in the heterokaryon. kar1 matings have been used to transfer YACs between 
yeast strains, but previous methods required specialized genetic backgrounds in the recipient strains and suffered from high rates of spurious 
chromosome transfer (Hugerat, Y., et al. 1994. 
Genomics 22:108). Plasmoduction with the universal donor strain only requires that the recipient strain be ura3, GAL+ and have another marker available for selection of the transferred plasmid. Counterselection against every donor chromosome also limits the amount of spurious 
allele transfer. The universal donor strain and the method of the invention are used to screen the 
yeast gene disruption 
library with plasmid-based 
dominant negative alleles of various genes.