The genome of haploid Saccharomyces cerevisiae contains two nonallelic sets of histone H3 and H4 genes. Strains with deletions of each of these loci were constructed by gene replacement techniques. Mutants containing deletions of either gene set were viable, however meiotic segregants lacking both histone H3 and H4 gene loci were inviable. In haploid cells no phenotypic expression of the histone gene deletions was observed; deletion mutants had wild-type growth rates, were not temperature sensitive for growth, and mated normally. However, diploids homozygous for the H3-H4 gene deletions were slightly defective in their growth and cell cycle progression. The generation times of the diploid mutants were longer than wild-type cells, the size distributions of cells from exponentially growing cultures were skewed towards larger cell volumes, and the G1 period of the mutant cells was longer than that of the wild-type diploid. The homozygous deletion of the copy-II set of H3-H4 genes in diploids also increased the frequency of mitotic chromosome loss as measured using a circular plasmid minichromosome assay.

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