Cells use specific mechanisms such as histone chaperones to abrogate the
inherent barrier that the nucleosome poses to transcribing polymerases. The
current model postulates that nucleosomes can be transiently disrupted to
accommodate passage of RNA polymerases and that histones H3 and H4 possess their
own chaperones dedicated to the recovery of nucleosomes. Here, we determined the
crystal structure of the conserved C terminus of human Suppressors of Ty
insertions 2 (hSpt2C) chaperone bound to an H3/H4 tetramer. The structural
studies demonstrate that hSpt2C is bound to the periphery of the H3/H4 tetramer,
mimicking the trajectory of nucleosomal-bound DNA. These structural studies have
been complemented with in vitro binding and in vivo functional studies on
mutants that disrupt key intermolecular contacts involving two acidic patches
and hydrophobic residues on Spt2C. We show that contacts between both human and
yeast Spt2C with the H3/H4 tetramer are required for the suppression of H3/H4
exchange as measured by H3K56ac and new H3 deposition. These interactions are
also crucial for the inhibition of spurious transcription from within coding
regions. Together, our data indicate that Spt2 interacts with the periphery of
the H3/H4 tetramer and promotes its recycling in the wake of RNA polymerase.
Links
