Normal stoichiometry of histone dimer sets is necessary for high fidelity of mitotic chromosome transmission

To identify gene products that function stoichiometrically in mitotic chromosome transmission, genes were cloned on high copy number plasmids and transformed into yeast cells, and the transformants were examined for an increase in the frequency of mitotic chromosome loss or recombination resulting from the gene imbalance. When either pair of the yeast histone genes H2A and H2B, or H3 and H4 was present on high copy number plasmids, both chromosomes V and VII exhibited an increased frequency of chromosome loss. The rate of chromosome loss was not elevated when the histone genes were present on single copy plasmids, when their transcription from high copy plasmids was repressed, or when frame-shift mutations were present in the coding sequence. This method for the identification of genes circumvents some of the limitations of traditional mutational analysis and yields the cloned gene.