The conformational flexibility of the C-terminus of histone H4 promotes histone octamer and nucleosome stability and yeast viability

To investigate the functional consequence of this structural change in histone H4, we restricted the available conformations of the H4 C-terminus and analyzed its effect in vitro and in vivo in Saccharomyces cerevisiae. One such mutation, H4 G94P, had modest effects on the interaction between H4 and Asf1. However, in yeast, flexibility of the C-terminal tail of H4 has essential functions that extend beyond chromatin assembly and disassembly. The H4 G94P mutation resulted in severely sick yeast, although nucleosomes still formed in vivo albeit yielding diffuse micrococcal nuclease ladders. In vitro, H4G4P had modest effects on nucleosome stability, dramatically reduced histone octamer stability, and altered nucleosome sliding ability.The functional consequences of altering the conformational flexibility in the C-terminal tail of H4 are severe. Interestingly, despite the detrimental effects of the histone H4 G94P mutant on viability, nucleosome formation was not markedly affected in vivo. However, histone octamer stability and nucleosome stability as well as nucleosome sliding ability were altered in vitro. These studies highlight an important role for correct interactions of the histone H4 C-terminal tail within the histone octamer and suggest that maintenance of a stable histone octamer in vivo is an essential feature of chromatin dynamics.The nucleosome is the repeating unit of chromatin, which organizes all eukaryotic genomes [1]. As a complex of DNA with an octamer composed of two copies each of four histone proteins, the central histone H3/H4 tetramer wraps the central 80 base pairs (bp) of DNA, and the flanking H2A/H2B dimers wrap the remaining 67 bp of the DNA into a compact structure [2-4]. This structure is repressive to most, if not all, DNA dependent processes. For example, both nucleosome assembly and disassembly are critical for proper DNA replication, repair, and transcription [5,6]. These processes are regulated by a vast array of chromatin remodeling