In Vivo Deletions
The histone H3-H4 deletion constructs were introduced into the chrumo-
1. Abbreviation used in this paper: ARS, autonomously replicating segment some using the shuttle vector YRpl4 and a protocol as previously suggested (23, 48). The YRpl4 vector carded the SUP11 ochre suppressor and thus the integrated plasmid could be detected by suppression of ade2-101 in the host yeast. This permitted the construction of a diploid that was homozygous for the integration of the shuttle plasmids at the histone loci. Diploid strain YP3 was transformed to UP, A + with YRpl4 plasmid derivatives conraining the copy-I or copy-II deletion constructs. The transformants were pink as a result of partial suppression of the homozygous diploid ade2-101
alleles by one copy of the SUPll plasmid gene. Isolates were sporulated and tetrads were dissected to yield two red and two white spore colonies. White segregants were the result of complete suppression of the single haploid ade2-101 allele by the plasmid SUP11 gene. The white colonies were screened for isolates that carried the YRpl4-histone deletion plasmids integrated at the histone loci by Southern blot hybridization analysis. From this set, a and Figure 1. Organization of the histone H3-H4 gene loci. The locations of the historic H3 and H4 genes are shown for the two nonallelic loci with respect to cloned DNA fragments. The positions of an unrelated gene SMTI, and the autonomous replication element, ARS, are also shown for the copy-I locus. The extents of H3-H4 gene deletions are indicated by the hashed bars over each of the maps. Each deletion removes the entire transcribed region for the H3 and H4 genes, For the copy-I locus, the SMTI gene was left intact but the ARS element was removed. 0t haploids were mated to give white diploids homozygous for the integrated
shuttle plasmid. The white homozygous diploids were grown on YPD plates containing1.5 M ethylene glycol to select for those cells in which the integrated shuttle vector had excised (13, 42). These cells gave pink colonies because of the loss of one copy of SUPH. Pink diploids were again sporulated and tetrads dissected to give two red and two white spore colonies. The red colonies identified meiotic segregants that inherited the chromosome in which the shuttle plasmid had excised. Genomic DNA from these red isolates were
screen by Southern blot analysis to detect the cells in which the excision of the shuttle plasmid left the histone gene deletion in the chromosome.