Histone octamer functionin vivo: mutations in the dimer-tetramer interfaces disrupt both gene activation and repression

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Abstract
Within the core histone octamer each histone H4 interacts with each H2A–H2B dimer subunit through two binding surfaces. Tyrosines play a central role in these interactions with H4 tyrosines 72 and 88 contacting one H2A–H2B dimer subunit, and tyrosine 98 contacting the other. To investigate the roles of these interactions in vivo, we made site‐directed amino acid substitutions at each of these tyrosine residues. Elimination of either set of interactions is lethal, suggesting that binding of the tetramer to both dimers is essential. Temperature‐sensitive mutants were obtained through single amino acid substitutions at each of the tyrosines. The mutants show both strong positive and negative effects on transcription. Positive effects include Spt‐ and Sin‐phenotypes resulting from mutations at each of the three tyrosines. One allele has a strong negative effect on the expression of genes essential for the G1 cell cycle transition. At restrictive temperature, mutant cells fail to express the CLN1, CLN2, SWI4 and SWI6 genes, and have reduced levels of CLN3 mRNA. These results demonstrate the critical role of histone dimer–tetramer interactions in vivo, and define their essential role in the expression of genes regulating G1 cell cycle progression.